Signals--Interlocking--Block Telegraph and Electric Train
Staff Instruments.
Signals.--Railway tradition alleges that on one of the early lines
opened for passenger traffic, the precautions for public safety were
considered to have been fulfilled by providing a man on horseback to
ride along the track between the rails in the front of the locomotive
engine, to give warning to persons strolling on the line, and to check
the advance of the train when necessary. A very short experience of
this method of working proved that the full capabilities of the
locomotive could not be obtained from a restricted speed of seven or
eight miles an hour, and a more comprehensive system of signalling had
to be devised. By fencing in on both sides of the line, the public
were prevented from making a general highway or promenade along the
railway, and the problem was reduced therefore to the signalling for
the trains alone.
Flags of different colours, held by flagmen stationed at suitable
places, answered the purpose for a while, or so long as the authorized
running speed did not prevent the train being brought to a stand after
sighting a flag warning the engine-driver to stop. As speeds were
increased, a longer or more distant view of signals became imperative,
and tall posts, or semaphore signals, were introduced. Well-defined
blades or discs placed on high posts were easily worked from the
ground-level, and could be seen for long distances, thus enabling the
trains to be controlled or brought to a stand before reaching the
signal. The efficiency of the principle once recognized, improvements
and additions were made from time to time, until we have the simple
acting tall semaphore signal so universally in use at the present
time. The position of the signal arms or blades in the daylight, and
the colours shown by the lamps at night, form the code of signals for
the proper working of the train service; and as the signal arms and
lamps are both worked simultaneously by the same gearing, it is only 314
necessary to light the lamps to put the signals in complete condition
for night-working. For some years, when the traffic was small, with
trains at low speeds and at considerable intervals, one double-arm
semaphore signal-post at a station was made to serve for all purposes;
but as the train service became more frequent and more rapid, it was
found that another semaphore or tall post signal, was necessary to
give warning to the engine-driver some distance back before reaching
the station or _home signal_. More particularly was this necessary at
those stations where it was not intended that every train should stop.
This new signal, called the _distant signal_, very soon came into
general use. It was placed at distances varying from 400 to 800 yards
away back from the station or home signal, and was worked by a long
strained wire extending from the distant signal to a ground-lever
placed near the home signal, the levers for these distant signals and
home signals being thus near together and under the control of one
man. More recently it was found necessary to introduce another
important wire-worked signal called a _starting signal_, which is
placed at the outgoing or departure end of the passenger platforms,
lines, or station sidings, to prevent any train or engine starting or
proceeding on its journey until such starting signal is lowered to
indicate that the line is clear.
These simple, independent, hand-worked semaphore signals did good
service for many years, but being independent and in no way physically
connected with one another at junctions, or stations, or with the
switches they were intended to control, it was quite possible for
mistakes to arise where everything depended upon the accuracy and
prompt decision of the signalman. The possibility that such mistakes
could occur, and the certainty that they actually did occur, and too
often with most disastrous results, led gradually to the grouping and
interlocking of a large number of signal levers and switch levers
together in one signal cabin. The advantages of the concentrating and
interlocking of signals and switches are twofold. In the first place,
one man in the signal cabin can work and control the levers for a
large number of switches and signals, where formerly several men were
required to be located at various places in the station-yard; but the
second, and by far the most important advantage, is that with proper
interlocking arrangements it is practically impossible to give
conflicting signals.
With a modern interlocking frame, and assuming the normal position of 315
all the signals to be at _danger_, then before a signal can be
lowered for an approaching engine or train all the switches and
corresponding signals, from any lines or sidings connecting with the
line to be signalled _clear_ must first be set so as to prevent
any engine or train coming out of such connecting lines or switches on
to the line to be made clear. In a similar manner, before the points
and signals can be set to permit an engine or train to pass from a
siding on to the main line all the necessary signals must first be set
to _danger_ to prevent the approach in either direction of any
engine or train on the main line about to be occupied. The mechanical
arrangements of the interlocking frame are so exact and complete as to
effectually prevent any but the proper combination being made. An
untrained or inexperienced signalman might inadvertently attempt to
pull over a wrong lever, only to find it securely locked and immovable
under the control of other levers. The proper sequence of levers must
be made, and the accurately adjusted mechanism automatically prevents
mistakes which formerly occurred with the old hand-worked signals from
the oversight or confusion of the signalman.
The interlocked switches or points are worked from the signal-cabin by
light wrought-iron tubing (termed rodding) or channel-shaped iron bars
supported on fixed iron rollers, and the signals by galvanized wires
running over light pulleys. Modern signals are always weighted at the
signal-post, so that in the event of the breaking of the pulling-wire
they will fly back to their normal position of danger.
The facility and precision secured by the interlocking machinery
enabled other valuable accessories to be introduced for the more
complete signalling and protection of train-working. Amongst these may
be mentioned the facing-point bolt-lock and rocking-bar,
signal-detectors at points, and throw-off or trap points.
With the old-fashioned hand-worked switches the man standing alongside
could see whether the sliding-rails were properly closed, and also
when the last vehicle of the train had passed over them; but when
important main-line-facing switches or points are worked by rodding
from a signal-cabin some distance away, it is necessary to have some
reliable means to ensure that the sliding-rails are actually brought
close home, and also to prevent the switches being moved again until 316
the entire train has passed over them. A set of switches may be
carefully made and work well, but it is quite possible for some
fracture or obstruction, to intervene and prevent them closing
properly. If a train or engine were passing through them in a trailing
direction, as indicated in Fig. 345, the wheels would most probably
force the sliding-rail home, and no disturbance would arise. If,
however, the train were coming in the opposite or facing direction,
the chances are that some of the wheels would take one road and some
the other, and cause a derailment. The same casualty would occur if
the switches were moved during the passage of the train.
To guard against the above contingencies, the facing-point bolt-lock
and rocking-bar have been introduced. The system is applied in various
forms, but the arrangement shown in Fig. 480 will explain the
principle generally.
A strong casting, A, is securely bolted to the top of the sleeper
carrying the chairs on which rest the point ends of the sliding-rails.
This casting has an internal groove or chamber formed for its entire
length from C to D, as indicated by the dotted lines, and in which
slides the locking-bolt B. The point ends of the switch or
sliding-rails are connected by the transverse rod E, which is forged
into a vertical bar form for that portion of its length, which passes
through the opening, F, prepared for it in the casting A. In this
vertical bar a hole or slot is cut to correspond to the exact size of
the locking-bolt B, and at a distance to suit the sliding-rails when
pulled over to their properly closed position. This locking-bolt, B,
will not pass through the hole in the vertical bar until the
sliding-rails are quite close home, and when once through the hole the
sliding-rails cannot be moved until the locking-bar is withdrawn. In
some cases two holes or slots are cut in the vertical bar to enable
the points to be bolt-locked for both directions.
The rocking-bar is designed to prevent the withdrawal of the
locking-bolt before all the vehicles have passed over the points.
[Illustration: Fig. 480, 483, 481, 482]
This rocking-bar consists of an angle iron or tee-iron bar of a length
equal to the longest wheel-base of the rolling-stock, and is carried
on short pivoted arms working in cast-iron or wrought-iron brackets
secured to the rails as shown in Fig. 481. The pivoted arms have a
movement backward or forward, and when at either the one or the other
extremity, the upper surface of the rocking-bar is sufficiently below 318
the top of the rail to be well clear of the flange of any passing
wheel; but while changing from the one to the other position, and when
the pivoted arms are vertical, or at half-stroke, the upper surface of
the rocking-bar is about level with the top of the rail, and right in
the pathway of the wheel-flange. It is evident, therefore, that when
the pivoted arms are set in the forward or backward position, and one
of the wheels of a train or vehicle has passed on to the rail over the
rocking-bar, the latter cannot be changed or raised and pulled over to
the opposite extremity so long as any one of the wheels of the train
or vehicles remain over the rocking-bar.
As the same ground-crank which pulls over the pivoted arms from
backward to forward also withdraws the locking-bolt B, the latter is
thus held securely in the hole or slot of the transverse rod, E,
until all the wheels of the train have passed off the rocking-bar. The
operation of changing the points from one road to another is very
simple. By means of the rodding G, worked by a lever in the
signal-cabin, the locking-bolt B is first withdrawn from the slot;
the points are then pulled over into the reverse position by the
rodding H, and the locking-bolt B is again set back into one of
the slots by the rodding G. Sometimes, for economy, the points,
bolt-lock, and rocking-bar, are all three worked by one lever in the
signal-cabin, and one set of rodding on the ground, as shown in Fig.
482; but the arrangement is neither so perfect nor so secure as that
shown in Fig. 480. Where there are two sets of rodding and gearing,
the failure or breaking of either of them prevents the complete
combination being made, and indicates at once to the signalman that
something is wrong; but when there is only one set of rodding a
breakage may occur without giving any tangible evidence to the
signalman of the defect, and he may proceed to pull over his signal
lever in ignorance that the points have not been properly made and
bolted. To avoid an accident taking place from the failure of either
rodding or gearing, the signal-detector has been devised, so as to
prevent the possibility of pulling over the signal wire until the
points and locking-bar are both in their proper positions.
The signal-detector is applied in several forms; the one shown in
Figs. 480 and 483 will explain the principle on which its efficacy
depends. A transverse rod, I, attached to the sliding-rail, extends 319
out beyond the rails, and is formed into a flat bar or plate, J,
sliding through the guide-holes K, K in the casting L. Short
upright levers, M and N, work on trunnions fixed in the casting,
and to M and N are attached the wires leading from the
signal-cabin and continuing on to the signal-posts, as shown in
elevation in Fig. 483. Two slots are cut in the plate J to receive
the curved arms of the levers M and N when they are drawn
downwards to pull off the corresponding signals. Neither of the
levers, M or N, can be drawn over unless there is a slot
immediately under the curved arm into which it can enter. When there
is solid plate under a curved arm, the short lever cannot be pulled
over, and the signal therefore remains at danger. The slots in the
plate J are spaced so that one will be brought into position for one
of the curved arms, when the points are close home for the main line,
and the other slot for the other curved arm, when the points are set
for the branch line or siding. The two slots cannot be under the two
curved arms at one and the same time, as one of the signals
corresponds to the main line and the other to the branch line or
siding.
In some forms of signal-detector the transverse rod I is joined on
to a vertical bar which slides through guide-holes in a casting
something similar to the arrangement shown in the casting L.
Longitudinal guide-holes, parallel to the line of rails, are made in
the casting a little above the transverse rod-bar, and through the
longitudinal guide-holes slide two vertical bars which are attached
to, and form part of, the wire connections to the two signals. The
wire bars have each a small tongue or rectangular fin forged on to the
under side of the bar, and there is one corresponding channel cut in
the transverse rod-bar. When the switches are properly closed in one
position, the channel cut of transverse bar will be opposite one of
the wire bar fins, and will allow one of the signals to be pulled over,
but the other wire bar cannot be moved. The closing of the switches in
the reverse position moves the channel cut so as to allow the other
wire bar to be pulled through, but as there is only one channel cut in
the transverse bar, only one signal can be pulled over for each
position of the switches.
[Illustration: Fig. 484, 485, 486, 490, 491, 487, 492, 488, 493, 496,
489, 494, 495, 497]
Throw-off or _trap points_, are introduced to throw an engine or train
off the rails of a siding on to the ballast, and so avoid a collision
with any other train which may be standing or passing on the line of
rails with which such siding forms a connection. Fig. 484 is a diagram 321
sketch of the arrangement, in which the main-line points are indicated
by the letter A, and the trap points by the letter B; one series of
rodding actuated by one lever in the signal-cabin works both the
main-line points and the trap points at the same time and by the same
movement. The connections are so made that when the points A are set
for the passage of trains on the main line, the trap points B are set
open to throw off on to the ballast, as shown in Fig. 484; and when
the main-line points A are set to allow a train to pass from the
siding on to the main line, the trap points B are closed, as shown on
Fig. 485. A disc or other signal, worked or interlocked with the
points, is placed near B to notify the engine-driver when he may pass
out of the siding on to the main line; but should he from any cause
proceed before the points are properly set and the corresponding
signal given, his engine would run off at the ends of the rails C, C,
and be derailed on to the ballast. The inconvenience caused by such
derailment would be trifling compared with what might result from a
collision with a train standing or passing on the main line. In some
cases the siding is continued onwards for a considerable distance from
the trap-point rails C, C, as indicated by the dotted lines D D, and
terminates with a dead end. When this arrangement can be adopted,
derailment is obviated, and the engine is brought to a stand by a
buffer-stop at the end of siding. On no account should trap points be
placed close to the top edge of a high embankment, or up to the
abutment or wing walls of an under-line bridge, where an engine
running through them accidentally might fall down a considerable
height, and cause serious results. All sidings joining on to main
lines should be trapped as above described, and when properly
signalled and interlocked in the signal-cabin, the traffic-working can
be carried on with increased facility and security.
Fig. 486 is a sketch of an average sample of an ordinary single-arm
wooden signal-post, with signal-arm, lamp, spectacles, ladder, and
gearing complete for wire connection to signal-cabin. When the arm
stands out in the horizontal position, representing the _danger_ or
stop signal, the red spectacles will be in front of the lamps, and
will show a red light to an approaching train. When the arm is
lowered, as indicated by the dotted lines, the second spectacle will
be in front of the lamps, and will show either a white or green light
(according to the accepted code) as an _all-right_ signal for the 322
train to proceed. For many years a white light was adopted for the
all-right signal, but latterly, to prevent confusion with other white
lights about a station, there has been an increasing disposition to
use a green light as an all-right signal. Several railway companies
have already effected the change, and others have arranged to follow
their example. The counter-weight W keeps the signal-arm to the
danger position, except when it is raised by the pulling over of the
signal-wire from the signal-cabin working over the pulley P. Should
the wire break when being pulled, the weight W falls down to the
stop-plate, and the signal-arm rises to danger. The signal-posts may
be of wood, wrought-iron, steel lattice-work, or cast-iron.
The arms of _distant signals_ should be cut to a fish-tail shape, as
in Fig. 487, to distinguish them from other signals. Goods-line
signals should have a thin sheet-iron ring, as in Fig. 488. Sometimes
purple glass is used instead of red glass for the spectacles of goods
signals. Letters or numbers may be attached to signal-arms to signify
the lines or sidings to which they correspond. Special signals are
sometimes made with the arm working on a centre pin, as in Fig. 489.
At junctions or places where two or three signals have to be fixed
near together, it is customary to carry them on a bracket signal-post,
as in Figs. 490 and 491. The former represents the home signals at an
ordinary junction, the taller signal being for the main line and the
lower one for the branch line. Fig. 491 shows the home signals at a
junction where there is one line turning out of the main line to the
left and another to the right. The taller signal in this case also
serves for the main line and the two lower signals for the branch
lines.
In important station-yards, where there are a large number of lines
and sidings running side by side, it is not always convenient or
possible to place the respective signal-posts in suitable positions
between the lines. To overcome the difficulty, the signals are erected
on light overhead lattice girders, as shown in Fig. 492. In some
cases, for want of a better position, or to obtain a more
comprehensive view of the lines and signals, the signal-cabin is built
on lattice girders, as in Fig. 493.
Ground or _disc signals_ are fixed at the ground-level, and are worked
in conjunction with trap points or outlet switches from sidings. In
some cases they are worked direct by a connecting-rod from the 323
switches, and serve merely as indicators to show whether the switches
are lying for or against an engine passing out of the siding. In other
cases they are worked independently from the signal-cabin by a
separate lever and wire connection, the interlocking being so arranged
that the lever working the switches must be pulled over before the
lever working the disc signal can be moved. In one type the disc
signal is fixed to a short vertical axis, as shown in Fig. 494, and by
means of a cranked arm is made to rotate a quarter of a circle, so as
to exhibit either a stop or advance signal according to the position
in which the switches are lying. In another type, the lamp is fixed,
and the red disc, with a red glass in the centre, is made to assume a
horizontal or vertical position by a rod and crank, as shown in Fig.
495.
A simple arrangement of rodding and rollers for switch connections is
shown in Fig. 496, the number of sets of rodding being determined by
the number of connections to be made. Fig. 497 is a rodding
compensator, to compensate or adjust for the difference in length of
the rodding arising from variations in the temperature. The
compensator may be used either vertically or horizontally, according
to space or circumstances.
Strong wrought-iron or steel cranks of different angles will be
required when changing the direction of the rodding, or connecting to
switches and facing point-locks. They must be firmly secured to strong
timber framework well bedded in the ballast. For cranks working
switches and bolt-locks, it is better to use extra long timbers under
the rails instead of the ordinary sleepers. Cross-pieces can be bolted
to the ends of the long timbers, and the cranks placed practically on
the same timbers carrying the permanent way. By this means the rails
and cranks can always be maintained in their proper relative positions
as to distance, line, and level.
Without a large series of diagrams it would be impossible to
adequately describe the extent of signalling and interlocking required
at large terminal stations and important roadside stations, but one or
two simple examples may serve to illustrate the general principles.
[Illustration: Fig. 498, 499]
Fig. 498 represents the modern grouping of signals considered
necessary at an ordinary double-line junction, showing all the signals
at their normal or _danger_ position. The numbers marked on each
indicate the numbers of the levers in the interlocking frame of the 325
signal-cabin. Four distinct sets of trains have to be dealt with at
this class of junction, and the interlocking must be so arranged that
when the signals are lowered for the advance of any one train, no
conflicting signals can be given to any other train.
Assuming a train approaching from A, which has to continue on the main
line past B on towards C, then the levers in the signal-cabin must
first be pulled over to set the points 9 and bolt-lock 8 in proper
position for the main line; and this operation will release the levers
which have to be pulled over to lower the signals 5, 4, and 6, but at
the same time will lock, and prevent the pulling over of the levers or
lowering of the signals 2 and 1 for a train from A to B and D, or of
the signals 14 and 15 for a train from D to B and A. The levers will,
however, be free to pull over for setting the points 12 and lowering
the signals 16, 17, and 13 for a train on the main line from C to B
and A.
In a similar manner, assuming a train approaching from D, which has to
continue up to the main line at B and on towards A, then the lever in
the cabin must first be pulled over to set the trailing points 12 in
proper position; and this operation will release the levers which have
to be pulled over to lower the signals 14 and 15, but at the same time
will lock, and prevent the pulling over of the levers or lowering of
the signals 5 and 4 for a train from A to B and C, or of the signals
16 and 17 for a train from C to B and A. The levers will, however, be
free to pull over for setting the points 9 and bolt-lock 8 and
lowering the signals 2 and 1 for the passage of a train on to the
branch line from A to B and D.
For a train from C to B and A, the levers 12, 16, 17, and 13 would be
required, and these would lock levers 14 and 15, and prevent the
approach of any train from D to B, but they would leave free the
levers necessary either for a train from A to B and C, or for a train
from A to B and D, but only one of them at a time, the setting of the
one series locking the other series.
A train from A to B and D would require the proper setting of the
points 9, bolt-lock 8, and signals 2, 1, and 3; and these would lock 5
and 4, but would leave free the levers necessary either for a train
from C to B and A, or for a train from D to B and A, but only one of
them at a time.
The cross-over road from the UP to DOWN main line, near the letter B 326
on sketch, is only intended for use in case of break-down or
accidents, and the normal position of the points is to lie clear for
the passage of trains on the main lines. To use the cross-over road,
the whole of the signals must first be set to _danger_ before the
points 7 and 7 can be opened to permit the passage of an engine or
train from the one main line to the other.
The starting signals 6 and 3 should be placed sufficiently far away
that the longest passenger or goods train may stand between them and
the clearance points at G and E. These starting signals are of great
service to train-working at junctions. Supposing a main-line train
from A arriving at B before the section from B to C was clear, such
train could be brought to a stand at signal 6, and remain there while
another train from A was allowed to pass B, and proceed onwards
towards D; or a branch-line train from A to D could be brought to a
stand at 3, to allow a main-line train to proceed onwards from A to B
and C. The starting signal 13 should be placed well in advance of the
cross-over road to control anything passing from one line to the
other.
Fig. 499 shows the modern grouping of signals for an ordinary
single-line junction. The arrangement is almost practically the same
as for the double-line junction shown in Fig. 498, there being the
same four distinct sets of trains to be controlled, but not any
cross-over road. The signal-cabin is placed on the main line, a little
in advance of the facing points, and a well-fenced-in gangway, the
same height as the engine footplate, is carried out the proper
distance from the rails, on which the signalman can stand to hand over
or receive the train staff from the engine-driver when passing.
At stations and places where there are several sidings and lines
connecting with the main lines, at considerable distances apart, it
will be necessary to have two or more signal-cabins placed in suitable
positions, not only for expediting the working of the constant
shunting movements, but also to insure that there is a signal-cabin
within the regulation distance of all facing points on the main line.
So far as the main line is concerned, the interlocking of these cabins
must be connected, the one with the other, by slotting, or co-acting
gearing, in such manner that the cabin in advance shall always be able
to control the cabin in the rear in the lowering of the main-line
signals for an approaching train. Fig. 376 is a diagram sketch of a 327
typical double-line roadside station with two signal-cabins. The NORTH
cabin has to work the signals and points in connection with the
goods-shed, goods-sidings, and market branch, and the SOUTH cabin,
those in connection with the coal and cattle sidings; and each of the
cabins to work the signals and points of that portion of the main line
adjoining its own cabin. For siding working, each cabin is quite
distinct and independent of the other, but for main-line working the
lowering of the signals can only be effected by the joint operation or
co-acting of both cabins.
Assuming a train approaching from A to proceed in the direction
towards B, then, before the signalman in the NORTH cabin can lower the
UP home-signal C, the signalman in the SOUTH cabin must first pull
over his lever and release the slot which retains the signal C at
_danger_, and in doing so the levers in his own cabin will stand
locked, and prevent the lowering of the signal D, or opening of points
E to allow access from the sidings to UP main line. The cross-over
road F G will also be locked for main line clear. When the slot has
been released from signal C, the signalman in NORTH cabin can lower
the UP home signal C, but before he can pull over the lever for this
purpose he must first lock the points H, to prevent access from the
sidings to the UP main line, and also the points K L of the cross-over
road, to keep the main line clear. A similar operation has to be gone
through for a train approaching from B to proceed in the direction
towards A, when the signalman in NORTH cabin must first withdraw the
slot from the DOWN home-signal M before the signalman in the SOUTH
cabin can lower that signal. A small automatic disc is placed in the
cabin to indicate to the signalman when the slot has been withdrawn by
his colleague in the neighbouring cabin, and for facility of working,
the two cabins are usually placed in communication with each other by
telegraph or telephone.
At some stations similar to the above, where there is a very frequent
train service, with several of the trains running through without
stopping, it is the practice to have a second or lower arm to the home
signals C and M, as shown on the diagram, these lower arms being only
_pulled off_ for through or non-stopping trains, as an indication to
the engine-driver that the line is clear in the section ahead.
In addition to the leading signals shown in the sketches, there 328
are shunting signals for the movement and marshalling of
trains--setting-back signals in connection with the making up of
passenger trains; taking on or off passenger carriages; or moving out
empty passenger carriages; and many other special signals which become
necessary for the working of a large and complicated train service.
The above simple diagrams will explain some of the principal
requirements to be kept in view when working out signalling
arrangements. Where the lines and sidings are very numerous, as at
important junctions and large terminal stations, the signalling
becomes very intricate, and may require three or four cabins, slotted
together in such manner that the necessary co-acting may be insured
for the proper controlling of the mainline signals. Many of these
signal-cabins contain a large number of levers, some of them having as
many as a hundred, and a few of them two hundred and forty levers, or
more, all of them so carefully arranged that no conflicting signal can
be given. Not only has much skill to be exercised in the accurate
adjustment of the interlocking machinery, but much study must be
devoted to determine the exact duty of every lever, for the locking or
releasing of other levers.
Signal-cabins may be built of stone, brick, or wood. They should be
roomy, well ventilated, and have abundance of light. Every
signal-cabin should be placed in the position from which the signalman
can obtain the best view of the signals and points under his charge.
The height of the cabin floor will depend upon any obstacles that may
intervene between the cabin and the signals, such as over-line
bridges, station roofs, buildings, or other obstructions. Sometimes
the floor has to be kept as low as five feet above rail-level, to
secure a line of sight under the over-line bridges; and in others the
floor has to be raised twenty, or even thirty feet above rail-level.
[Illustration: Fig. 500, 508, 501, 502, 506, 507]
Figs. 500, 501, and 502 show plan, transverse section, and elevation
of a signal-cabin suitable for a small roadside station. The lower
story and chimney-stack are of brick, and the upper story of wood,
with slated roof. There is room for an interlocking frame of twenty or
twenty-five levers, and space at the end of the cabin for the
block-telegraph instruments, or electric train-staff instruments. The
roof-work is open up to the slateboards, to obtain as much air
capacity as possible. In the transverse section a winch for working
mechanical gates is shown at the end of the interlocking frame. There 330
is a liberal amount of glass, and two or three sliding windows, which
the signalman can open to enable him to speak to the engine-drivers or
others during shunting operations. The lower story of the cabin can be
utilized for trimming lamps and keeping a small supply of coals and
other stores. When working after dark the lamps in the cabins should
be well protected by shades, to prevent the lights being seen by
engine-drivers, and mistaken for signals.
Interlocking.--There are several systems of interlocking, each of them
varying considerably in the form and mode of application, but all of
them having the same general object of securing or releasing the
necessary levers for each combination of signalling movements. A brief
description of one of the systems will explain the order in which the
movements have to be made, and the security which can be obtained by
the locking.
Figs. 503, 504, and 505, are sketches illustrating one of the types of
wedge and tappet interlocking. Each lever works on a fulcrum or pinion
as at A, and has a lower arm B for lifting the rods leading off to
points or signals, and an arm C to carry a counterweight when necessary.
Cast-iron braces D are placed at convenient distances between the series
of levers to carry the top frame E on which the lever floor casing
F is bolted. This casing is continuous from end to end of the locking
frame, with the exception of the narrow openings through which the
levers travel when moving backwards or forwards. The sleeve-block
G, resting in the depressed portions of the arc, retains the lever in
position. When taking hold of the main lever L, the signalman’s hand
draws the small side lever M, close to the main lever, and raises the
sleeve-block G sufficiently high to pass over the top of arc F, the
lever L can then be pulled or pushed over, and the block G will fall
into the depression at the end of the stroke when the hand is removed.
N is a tappet or thin flat bar attached to the main lever, and which
works backwards or forwards between the wedges in the wedge frame O.
The wedges move horizontally between guide pieces, and work either
singly or are connected by the lower slide bars to other wedges some
distance away on the frame according to the position of the levers
which have to stand or move in unison for the releasing or locking. A
strong cover is placed over the wedge frame to keep out the dirt.
[Illustration: Fig. 503, 504, 505]
Figs. 504 and 505 show plan views of four levers in a signal cabin 332
taken just above the level of the tappets. In Fig. 504, all the levers
are in their _normal_ or forward position, with the home and distant
signals at _danger_, and the facing points leading into loop or siding
lying for main line. Previous to the approach of a train on the main
line, the home and distant signals have to be lowered, and will
require the pulling over of levers 1 and 2; but these levers cannot of
themselves be moved, as the wedges P and Q are locked by the straight side
of lever 3. The operation would therefore be as follows:--points lever
4 being set in its normal position for the main line would remain
forward, lever 3 working the facing point bolt-lock would be pulled
over, and in doing so would move the wedge R to the right into the
recess of tappet of lever 4, locking that lever, and presenting the
recess of its own tappet ready to receive the wedge Q. Lever 2 can
then be pulled over, and will move the wedge Q to the right into the
recess of tappet of lever 3, and present its own recess for wedge P.
The pulling over of lever 1 completes the series, by moving the wedge
P over to the right into the recess of tappet of lever 2. Fig. 505
shows the positions of the tappets and wedges with the levers 1, 2,
and 3, pulled over to make the combination described. Upon
examination, it will be seen that levers 2, 3, and 4, are all securely
locked, the points cannot be moved, nor the facing point bolt-lock
withdrawn, nor the home signal changed until the lever 1 is pushed
over again into its normal or _danger_ position. To restore the levers
to their forward position, they must be set back in the reverse order
to which they were pulled over. To simplify the explanation, only four
levers are shown in the above sketches, but the principle is
constantly extended out to a very large number of levers, and in many
cases necessitates the introduction of several rows of wedges as
indicated by the dotted lines. In some instances a combination is
effected by pulling a certain lever only half over. In some systems
the preliminary action or spring handle locking is adopted, in which
the locking is actuated by the small side lever, similar to the one
marked M on Fig. 503. The advocates of this arrangement claim
increased security and precision in the interlocking, while on the
other hand it is alleged that the mechanism is rendered more
complicated without any corresponding advantage.
Detached Lock.--Sometimes there is in the vicinity of a railway station, 333
a siding which is too far away to be worked direct from a signal
cabin, and not sufficiently used to warrant a separate cabin. Such
sidings can be worked by a small ground frame opened or locked by a
special key attached to the interlocking machinery in the adjoining
signal cabin on a double-line railway, or attached to the train staff
on a single line.
Fig. 506 shows the arrangement applied to a double line with the
outlying siding turning out of the UP main line, the points lying in a
trailing direction for the running trains. Before the special and
_only_ key can be withdrawn from its seat in the interlocking frame of
the signal cabin, all the UP main line signals must be set to
_danger_, and cannot be moved from _danger_ until the key is restored
to its proper seat again. When the key is removed from the signal
cabin, it can be taken to the ground frame at A, inserted in the key
opening, and by turning it partly round, will release the bar which
locks the levers of the facing point bolt-lock and the points. When
these two levers are free the points can be opened, and vehicles moved
into or out of the siding B C, but the special key cannot be withdrawn
from the ground-frame A, until the points and facing point bolt-lock
are put back again into their normal position for main line working.
When the operations at the siding are completed, the special key can
be removed, and taken back to its proper place in the signal-cabin,
and ordinary working be resumed.
Fig. 507 shows the application of the detached locks on a single line,
and is a sketch of a portion of railway on which there is a small
station B, with a goods siding F G, where the traffic is too small to
require anything more than ground frames and detached locks. An
engine-driver before leaving the station A, receives a train staff,
which gives him possession of the line as far as C, including of
course the intermediate station B, and this staff he must carry with
him and hand over to the signalman on his arrival at the end of the
section at C. At each of the points D and E is placed a two lever
ground frame, similar to the one shown in Fig. 506, and attached to
the train staff is a key, which will operate either of the two ground
frames, but only one at a time, as the key must be inserted before the
levers can be moved. When the train is proceeding in the direction
from A to C, it will be more convenient to shunt vehicles into or out
of the siding F G, by means of the points E, but when proceeding from
C to A, the points D will be more convenient. Whichever of the points 334
be used, they must be set, and bolt locked for the main line before
the train staff and its key can be withdrawn from the ground frame and
restored to the engine-driver. As the siding is _trapped_ at F and G,
it is impossible for any vehicles to be moved out on to the main line
except through the medium of the train staff and key. The same
arrangement of detached lock is equally available for a single siding
with only one set of points.
Electric Repeater.--It will sometimes occur that on account of a curve
or other obstacle, the arms and back lights of a distant or other
signal cannot be seen from the signal cabin, and it is necessary to
introduce an electric repeater. This little instrument consists of a
miniature semaphore signal fixed in a metallic box with a glass front,
and placed on a stand about a foot above the floor level immediately
in front of the signal lever for which it is intended to serve as an
indicator. Like the signal proper, the normal position of the
miniature semaphore is at _danger_, but when the signal lever is
pulled over in the cabin, the rod that pulls down the arm on the
signal post effects a contact with an electric circuit which lowers
the arm of the miniature semaphore at the same moment that the signal
arm proper is lowered, and gives visible indication in the cabin that
the signal is working. Fig. 508 is a sketch of one form of electric
repeater.
Detonators or fog signals are largely used in foggy weather and
snowstorms, when the out-door signals cannot be seen from an
approaching train. At such times the atmosphere is so dense, and the
surrounding objects so obscured, that an engine-driver is totally
unable to distinguish the usual landmarks which guide him on the
approach to a station or semaphore, and he might easily pass by a
signal unless he received an audible signal to indicate the position
of the one that is invisible. Detonators are usually made in the form
of a circular tin or metallic case about two inches in diameter, and
three eighths of an inch thick, with soft metal clips on opposite
sides for bending over and securing to the rails. The case is filled
with detonating powder, which is crushed by the first wheel passing
over it, and explodes with a loud report. It is customary to use these
detonators in pairs placed a short distance apart in case one of them
should fail to explode.
Fog-signalling regulations vary on different railways, but the 335
arrangements are generally carried out somewhat in the following
manner. During the prevalence of a fog or snowstorm, a fog signalman
is placed near each of the signal-posts to be protected, and is
supplied with a hand signal-lamp, hand-flags, and a packet of
detonators. So long as the arm of the signal-post at which he is
alongside stands at _danger_, he must keep two detonators on the rail
of that line which the signal controls, and also show a RED
hand-signal (hand-flag by day, and hand-lamp after dark) to the
approaching train. When the signal arm is lowered to show that the
line is clear for the passage of the train, the fog signalman must
remove the two detonators, and show a GREEN hand-signal (flag, or
lamp) to the approaching train. When an engine driver hears the report
of a detonator crushed by his engine, it is his duty to shut off steam
immediately, and bring his engine to a stand, after which he must
proceed very cautiously, until he receives further signals by hand or
otherwise, or receives the line-clear signal to continue on his
journey. Detonators are also of great service both in fine or bad
weather, in cases of a wash away, a failure of works, or obstruction
on the line, when a hand-signal may not be seen, but a detonator must
be heard.
Mechanical Gates.--Mechanical gates, worked and controlled from the
inside of a signal-cabin, are now very largely adopted for public road
level-crossings instead of ordinary hand-gates, opened and closed by a
gateman walking from side to side of the line across the rails. Being
worked from inside the cabin, they remove all possibility of the
gateman being struck by a passing train; they move simultaneously, and
can be opened or closed in very much less time than hand-worked gates,
which have to be moved one by one, and being interlocked with the
signals, the mechanical gates cannot be placed across the lines of
rails until the train-signals in each direction are set at
_danger_. When set for either train traffic or public road traffic,
the gates are held firmly in position by metal stops, rising out of
cast-iron boxes lying flush with the ground, and worked by a separate
lever in the signal-cabin.
Assuming the gates to be set for train traffic, and it is desired to
open them for the public road traffic, the first operation will be to
pull over the levers, and raise the signals in each direction to
_danger_, and thus release the stop-lever, which can then be pulled
over, to lower the gate-stops and allow the gate-winch to be turned,
and the gates moved round into correct position. The stop-lever must 336
then be set back to raise the stops and hold the gates secure. The
train-signals will be retained at _danger_ by the interlocking
gearing, and cannot be lowered until the gates are set back again
across the public road, and the gate-stops raised.
It is frequently urged that the celerity with which mechanical gates
can be swung round and closed across the public road, is in itself a
source of danger, and that persons preparing to cross the line might
be struck by a moving gate, unless they received a distinct warning
that such closing was about to take place. There is no doubt persons
have been struck by such gates when closing across the road, and heavy
claims for injuries have been decreed against railway companies, who
were unable to prove that the man in charge had called out or given
warning before moving the gates. To ensure that due and undeniable
warning shall always be given, a firm of signal-makers have patented
an appliance by which a powerful electric gong, fixed on the top of a
tall post close to the gates, is sounded automatically by the gate
machinery itself, and before the gates actually commence to move. As
previously described, the pulling over of the lever to lower the
gate-stops is the first operation to be performed whenever it is
necessary to change the position of the gates, and it is the pulling
over of this lever which actuates the apparatus, by bringing two
electric points into contact, and thus starting the ringing of the
gong or alarm. The gong continues to sound until the gates are moved
over, the gate-stops raised, and the stop-lever put forward again into
its normal position. The arrangement is very simple and very
effective, and being purely automatic must work as regularly as the
stop-lever. The tone and volume of the gong can be varied to suit
circumstances. The public soon become familiar with its sound, and
recognize its meaning.
[Illustration: Fig. 509, 510]
Figs. 509 and 510 give sketch plan and elevation of a set of
mechanical gates for a public road level crossing on a double line of
railway. The signal-cabin should be placed within a few yards of the
gates, to enable the man in charge to have a good view of the persons
and vehicles passing over the roadway. The underground gearing for
working the gates and stops, must be protected by iron or wooden
casing. The swinging portion of the wicket gates is closed, and held
by a separate lever. The gates shown on the sketch are for a crossing
on the square, but they can be equally well arranged for an oblique 338
crossing, and of widths to suit the locality.
Block-Telegraph Signalling.--However complete the outdoor signals and
interlocking at any station, they can only control the movement of
trains within their range, and something more is requisite to ensure
the safe working of the traffic over the long lengths of line between
stations. For some years a time-interval was allowed for the working
of trains following one another on the UP and DOWN lines of a
double line railway, no train being allowed to leave a station sooner
than a fixed number of minutes after a previous train had started in
the same direction. With this system there was always the risk that
the first train might be overtaken and ran into by the second, and
especially in the night time, or when the atmosphere was at all foggy.
The electric telegraph was then called in to assist in the
train-working, and brief telegrams were passed between the stations
announcing the departure and arrival of trains. The increased security
and convenience thus obtained led to the introduction of special
electric telegraph instruments, devoted to the exclusive duty of
train-working. These instruments, termed block telegraph instruments,
are now almost universally used on all double lines of railway, and
have largely contributed to the safe and efficient working of an ever
increasing traffic. They are made in various forms, but the object of
each is to ensure that before any train is allowed to start from, or
pass any station, the signalman at that station shall receive from the
signalman in the cabin in advance a distinct visible signal that the
line is clear, and free of any train up to the cabin in advance; and
also that after the train has been despatched, the signalman in the
rear shall be at once advised when the train has arrived at the
signal-cabin in advance. Fig. 511 is a sketch of one type of
block-telegraph instrument, in which the leading feature is the
miniature signal-post with its two arms, an arrangement which readily
appeals to the eye of the signalman as being so similar in form and
action to the fixed signals in the station. Each instrument is
supplied with a bell or gong, by which the adjacent signalmen can
communicate with each other, in accordance with a fixed code of
signals which defines the relative numbers of strokes of the bell or
gong, to represent certain regulation calls and answers. In the
signal-cabins of the intermediate stations, two block-telegraph 339
instruments are required, one for the section of the line to the left
hand of the cabin, and the other for the section to the right. At the
terminal stations only one instrument is required.
In the instrument shown in Fig. 511, the upper arm of the miniature
signal-post is coloured RED, and is moved by electricity through the
medium of the block telegraph instrument in the signal-cabin in
advance; and until this RED signal be lowered to the _line clear_
position by the signalman in the cabin in advance, no train must be
allowed to start from or pass the cabin in the rear. The lower
signal-arm coloured WHITE is lowered by the plunger A on its own
instrument by the signalman in charge, and at the same moment lowers
by electricity the upper or RED arm of the block-telegraph instrument
in the signal-cabin at the other end of the section. The lower or
WHITE arm is thus restricted to the signals sent away from the
signal-cabin, while the upper or RED arm is restricted to signals
received in the signal-cabin. In the centre there is a round handle B,
which rotates a circular disc inside the instrument, and on this disc
are painted three distinct train inscriptions, only one of which can
be seen at a time through the glazed opening. One inscription has the
words ALL CLEAR painted in black letters on a WHITE ground; another
has the words TRAIN ON LINE painted in white letters on a RED ground;
and the third has the words TRAIN OFF, BUT SECTION BLOCKED painted in
black letters on a GREEN ground. The instrument is considered to be in
its _normal_ position when the GREEN inscription is in view, and both
the miniature signal-arms raised to _danger_.
Fig. 512 represents a portion of double line divided out into sections,
or working blocks, between the stations B, C, and D. Each station is
provided with the necessary block-telegraph instruments, and the usual
distant, home, and starting semaphore signals.
[Illustration: Fig. 511, 512, 513]
Fig. 513 is a diagram sketch showing the pair of instruments as they
stand on the instrument-tables in the signal-cabins B and C, where B^2
and C^1 are the instruments which work together for the block section
BC. Supposing a DOWN train proceeding from A in the direction of F,
and approaching the signal-cabin of the block station at B, the DOWN
starting signal standing at _danger_; then by the code of signals on
the bell or gong the signalman at cabin B would communicate with the
signalman at cabin C, to obtain _line clear_, so as to allow the 341
approaching train to proceed on to C. If the previous train in the
same direction had already passed C, and there was not any obstruction
on the line, the signalman at C would give _line clear_ for the DOWN
train, and to do so he would turn his circular disc to show the WHITE
inscription ALL CLEAR, and then push in the plunger of his C^1
instrument, lowering the DOWN or white arm, K, of his own instrument
to the position shown by the dotted lines, which operation would at
the same moment lower by electricity the DOWN or red arm, G, of the
instrument B^2 in cabin B to the position of the dotted lines. The
signalman at B would then lower his starting signal, to allow the DOWN
train to proceed on towards C, and immediately the train had passed
the starting signal he would, by means of his bell or gong advise the
signalman at C that the train had entered the section, or block BC,
and the signalman at C would at once turn his circular disc to show
the RED inscription TRAIN ON LINE, and use his plunger to raise to
_danger_ the DOWN or white arm, K, of his own instrument, and at the
same time raise by electricity the DOWN or red arm, G, to danger in
the instrument B^2 in cabin B. The section BC would then remain
blocked until the DOWN train had arrived, or passed the station C,
when the signalman there would, by means of his bell or gong advise
the signalman at B that the DOWN train had passed out of the section,
and would turn his circular disc to show the GREEN inscription TRAIN
OFF, BUT SECTION BLOCKED. Both instruments would then be in their
_normal_ positions, with the arms raised to danger, and ready for
further train operations. In a similar manner for the UP-line trains
on the section or block between C and B, the signalman in B cabin
would turn his circular disc, and use his plunger to lower the UP or
white arm, H, in his own instrument, B^2, and at the same moment lower
by electricity the UP or red arm, I, of the instrument C^1 in cabin C,
the other operation for train on line and train off being carried out
for the UP train in the same routine as for the DOWN train. The
outdoor fixed signals, or distant home and starting semaphore signals,
have all to be worked to correspond to the block telegraph signals,
and as the latter are always received well in advance of an
approaching train, it follows that when the line is clear, the outdoor
signals can be lowered so as to allow a through or non-stopping train
to pass a block-telegraph station at full speed.
Where the traffic is moderate, it may be sufficient to have 342
block-telegraph instruments at each of the stations, but with a very
frequent train service it will be found necessary to divide the line
into shorter sections, and erect signal-cabins and block-telegraph
instruments at intermediate points between stations.
The code of bell or gong signals is extended to include various
matters in connection with the train-working. For example, when a DOWN
train is passing cabin B at full speed, the signalman may observe that
there is something wrong--a carriage or waggon on fire, a tail-lamp
missing, or other irregularity. It is too late to stop the train with
his own signals, but by means of his bell or gong he can call upon the
signalman in cabin C to stop and examine the train, and the DOWN
distant and home signals at C can be raised to _danger_ before the
train reaches the cabin at C.
In every block-telegraph signal-cabin there is a train-book in which
the signalman has to write down the time and description of every
arriving or passing train, and, as this book lies before him, he has a
complete record of the train-working, with the particulars of the
exact times when the _line clear_ signals were given, and also when
the train arrived or passed his signal-cabin.
To guard against the possibility of a signalman inadvertently giving
_line clear_, or allowing another train to pass his cabin before the
previous train had reached the signal-cabin in advance, some railways
have adopted the lock and block system. By this arrangement the
starting signal at any cabin is electrically and mechanically locked
from the cabin in advance, and can only be released or lowered by the
action of the outgoing train itself when passing over a treadle or
other appliance connected with the rails of the running-line at the
signal-cabin in advance. This method practically gives the train the
complete control of the section; and any signalman attempting, in
error, to lower his starting signal would find it to remain fixed to
_danger_ and immovable, until released by the arrival of the
train at the advance cabin.
Train-staff for Single Line.--When there is only a single line of
railway for both an UP and DOWN train-service, very definite
precautions must be adopted to prevent the meeting or collision of
trains travelling in opposite directions. Where the piece of single 343
line is short, and can be worked by one engine in steam, or two
coupled together, no collision can take place, as the train-service
will be limited to the one train moving backwards and forwards over
the section; but with a long length of single line, including a large
number of stations, necessitating several trains, some clear and
comprehensive regulations must be introduced. For a long time the
simple train-staff was found to give the desired security; there was
only one staff for each pair of adjoining staff-stations, and no train
was authorized to run without the staff, and as the staff could only
be on one train at a time, the precaution against collisions was
looked upon as complete. These staffs, which were generally made of
brass, or other metal, were sufficiently large to be conspicuous when
placed in the stand prepared for them on the engine. They were
lettered to correspond to the stations to which they belonged, and
were made in different patterns to distinguish them for their
respective sections. No train was allowed to start from a station
until the engine-driver received from the station-master the proper
staff to authorize him to proceed to the next station, and on his
arrival there it was the duty of the engine-driver to hand over the
train-staff to the stationmaster of that place, and wait for another
train-staff to authorize him to proceed over the next section. So long
as the train service could be evenly arranged, and that there was
always an UP train to take back a train-staff which has been carried
out by a DOWN train, the simple staff worked most efficiently; but as
the traffic increased, and two or more trains had to be despatched in
the DOWN direction before one had to run in the UP direction, some
auxiliary arrangement had to be introduced. This was effected by
issuing train tickets, kept in a locked-up box, which could only be
opened by the key attached to the train-staff. A properly dated
train-ticket was handed to the engine-driver of the first DOWN train,
and, if necessary, a second train-ticket to the engine-driver of a
second DOWN train, and then the train-staff itself was handed to the
engine-driver of the third DOWN train. There were one or two serious
drawbacks to this train-staff and ticket-working. As there was only a
time interval between the starting of the trains, the one train might
overtake and run into the other with disastrous results. Again, a
second or third train, which was put down in the schedule, might be
withdrawn at the last moment, and the staff left behind at a station
when it was required at the opposite end of the section, thus causing 344
much confusion and delay. The ordinary electric telegraph could have
been utilized to assist in regulating these train movements, but it
was felt that a mere telegraph message was not sufficient to ensure
positive safety, and that something more tangible was required in the
shape of a staff, or token, without which no train should be allowed
to travel on a single line of railway. To meet this requirement, the
electric train-tablet, and the electric train-staff instruments have
been invented, each of them being so arranged that upon any one
section, or pair of instruments, a tablet or train-staff may be taken
out from the instrument at either end of the section, but when once
taken out, no other tablet or train-staff can be withdrawn from either
instrument until the first has been delivered and placed again in one
or other of the two instruments.
Figs. 514, 515, and 516 are sketches of an electric train-staff
instrument which has been very largely adopted on single lines, both
at home and abroad.
In a similar manner to the block-telegraph instruments for double
line, the electric train-staff instruments have each a bell or gong by
which the adjacent signalmen can communicate their calls and answers
in accordance with a regulation code. In the signal-cabins of the
intermediate stations two instruments are required, one for the staffs
belonging to the section to the left of the cabin, and the other for
the staffs of the section to the right. At a terminal station only one
instrument is required.
[Illustration: Fig. 514, 516, 515, 517]
The head of the instrument contains the electrical and mechanical
locking apparatus which controls the withdrawal of a train-staff, or
is acted upon by its insertion. The circular name-plates and pointers,
together with the galvanometer in the centre, serve as indicators to
guide the signalmen in carrying out the various operations. The staffs
usually consist of thin steel tubes, solid at the ends, with metal
rings fixed upon them, as shown in the sketch, the number and position
of the rings varying according to the section or pair of staff
stations to which they belong; this difference in the rings
effectually preventing the possibility of one set of staffs being used
or inserted in either of the instruments of the adjoining sections.
The staffs rest normally in the long vertical slot A, with the rings
fitting in vertical grooves, which prevent the removal of any staff
except by passing it along the curved slot BC, and out by the opening 346
D, of large diameter. The electrical and mechanical locking apparatus
is placed at the curved slot, and until the locking-bolt, which stands
across the passage of the curved slot, be lifted by the joint
operations of the signalmen and their instruments at both ends of the
section, no staff can be withdrawn. When the instruments are standing
in their _normal_ position of “staffs in,” the signalmen can arrange
between them to withdraw a staff--say either from the NORTH cabin
instrument or from the SOUTH cabin instrument of the section, but only
from one of them; and the act of taking out that staff automatically
locks both instruments, and prevents the possibility of taking out any
other staff from either instrument until the staff already removed is
restored and inserted in one or other of the instruments. From the
above description it will be seen that the electric train-staff
instrument provides for the safe working of two or more trains
proceeding, one at a time, in the same direction over a section of
single line, each one being supplied with a train-staff, which must be
handed over at the end of a section before another staff can be issued
for a following train. Should the train-staffs accumulate in one
instrument, in consequence of more trains running in one direction
than another, a re-distribution of staffs is effected by the
authorized persons according to fixed regulations.
In the diagram sketch, Fig. 517, a piece of single line is shown
divided into sections or blocks, with loops or passing-places at the
stations. At the station E a train-staff taken out of the instrument F
serves for the section up to the instrument L at the station H; and on
the train-staff is a key which will open the detached locks on the
points of the small intermediate station, G, as described in Fig. 507,
in connection with the working of detached locks. At the station H the
engine-driver receives another staff from the instrument M, which
takes him to the instrument N at station K, and in like manner on this
staff is a key which will open the detached lock on the colliery
siding points at I. At stations H and K are shown loops, or short
pieces of double line, with platform to enable an UP train to cross or
pass a DOWN train. The distance apart of the electric train-staff
stations will depend greatly upon the number of the trains, and for a
frequent train-service it may be necessary to have the instruments at
every station, whether large or small. The electric train-staff is of
great advantage in the working of ballast or construction trains, as 347
a staff may be taken out of the instrument F at station E, which will
give possession of the section as far as station H, and when the
ballasting operations--which may be very near to E--are completed, the
train can return to E, and deliver the staff again to the instrument
F, instead of having to run the entire distance to station H. Although
carrying a train-staff, the engine-driver must approach stations
cautiously, and obey the fixed signals in the usual manner.