There are several fortunes to be made yet in improving it. For
instance, it is desirable, in descending long and steep grades,
that the brake-pressure should be just sufficient to control the
speed of the train, and should be steadily applied; otherwise
the descent will be by a succession of jerks which may become
dangerous. With the automatic the brakes must be occasionally
released to recharge the reservoirs, or when the speed of the train
is too much reduced; and it is difficult to keep a uniform speed.
So far, the means devised to overcome this difficulty and keep a
constant and light pressure on the wheels have been thought too
costly or complicated for general use. With hand-brakes long trains
are controlled by the brakes of but a few of the cars in any one
train. It follows that in the descent of grades the braked wheels
must often run for miles with the pressure as great as it can be
without sliding the wheels. The rim of the wheel is rapidly heated
by the friction of the brake-shoe, and the unequal expansion of
the heated and the unheated parts of the wheel causes a fracture.
This is why so many broken car-wheels are found at the foot of
grades--of all places the worst for such an accident to happen.
With "straight air," that is, with the pressure from the main
reservoir, or the air-pump, going directly to the brake-cylinder,
the engineer can apply the brakes to all the wheels of his train
simultaneously, and with great delicacy of graduation; and by
turning a three-way cock which is placed in the piping of each
car, the air can be used "straight." This is regularly done on
some mountain-roads. At summits the trains are stopped and the
brakes are changed from "automatic" to "straight." This practice is
dangerous, however, and is not approved by the best brake-experts,
for if a hose bursts, or through some other accident the air in
the train-pipe escapes, the brakes are useless. The automatic
arrangement by which a reduction of air-pressure in the train-pipe
applies the brakes, as previously explained, is much preferred,
although no entirely satisfactory means has yet been devised for
automatically regulating the air-pressure in the brake-cylinder.
There is not space here to enter into the history of the air-brake.
It was first practically applied to passenger trains in 1868.
The first great epoch in its subsequent development was the
invention, by Mr. George Westinghouse, Jr., of the triple valve.
The introduction of the triple valve at once reduced the time of
full application of the brake for a ten-car train from twenty-five
seconds to about eight seconds. This means, at forty miles an hour,
a reduction by more than one thousand feet in the distance in which
a train can be stopped. The next great epoch in the history of the
air-brake was made by the celebrated Burlington brake-trials of
1886 and 1887. These trials were undertaken by a committee of the
Master Car-builders' Association, to determine whether or not there
was any power-brake fit for freight service. For general freight
service the brake must be capable of arresting a very long train,
with cars loosely coupled, running at a fair average passenger
speed, without producing objectionable shocks in the rear of the
train. The two series of trials were carried out in July, 1886,
and May, 1887. The competing brake-companies brought to the trials
trains of fifty cars each, equipped with their devices. Skilled
mechanical engineers from various railroad and private companies
assisted both years. These trials were most exhaustive, and have
contributed more to the art of braking than any that preceded or
have followed them. The first year's trials developed the fact that
the air-brakes could not be applied on the rear of a fifty-car
train in less than eighteen seconds, whereas the head of a train
moving twenty miles an hour could be completely stopped in fifteen
seconds. The result was that disastrous collisions between the
cars of any one train were produced in the act of stopping. Men in
the rear cars were thrown down and injured, and much damage was
done to the cars. At the end of nineteen days the brake-companies
went home to work another year over the new problem. In 1887 they
reappeared on the same ground, and in eighteen days proved that
no simple air-brakes, as then operated, could prevent disastrous
shocks in a long train; but it was shown that by bringing in
electricity to actuate the air-valves, the application of the
brakes could be made practically simultaneous throughout the train.
Mr. Westinghouse, however, during the summer following, made such
modifications in the triple valve and in the train-pipe that he
succeeded in applying the brakes throughout a fifty-car train in
two seconds. That settled the matter. He at once equipped a train
of fifty cars, and in October and November, 1887, that train made
a journey of about three thousand miles, making exhibition stops
at various cities. The journey was a splendid and conclusive
demonstration that the air-brake is now a thoroughly efficient
and reliable contrivance for freight as well as for passenger
service. The result has been a very rapid application of the new
quick-acting brake to freight cars. The performance of this train
was to railroad men most impressive. A freight train of fifty cars
is about one-third of a mile long. To see such a train, running
forty miles an hour, smoothly stopped in one-third of its own
length, without shock or fuss, was an object-lesson that no one
could fail to understand or to remember. Some of the stops made by
this train will give a fair notion of the relative power of hand-
and air-brakes for quick stops. The following figures are averages
of stops made in six different cities. They give the distances run
in feet from the instant of applying the brakes till the train was
brought to a stand-still:
Feet.
Hand-brakes, 50 cars, 20 miles an hour 794
Air-brakes, 50 cars, 20 miles an hour 166
Air-brakes, 50 cars, 40 miles an hour 581
Air-brakes, 20 cars, 20 miles an hour 99
With twenty cars at twenty miles an hour even shorter stops were
made than those recorded above. In the Burlington trials the
hand-brake stops, with fifty-car trains at forty miles an hour,
were made in from two thousand five hundred to three thousand feet.
[Illustration: Dwarf Semaphores and Split Switch.]
The air-brake is somewhat complicated, but the complicated
mechanism is strong, has little movement, and is securely protected
from dirt and the elements. It is therefore little liable to
derangement. It is, however, becoming better understood that
brake-gear must be good, and employees carefully instructed in
the care and use of the air-brake to get its best results; and in
recent years two or three elaborate instruction-cars have been
fitted up for the education of the enginemen and trainmen.
Space does not permit more than an allusion to driver-brakes,
which are operated by steam and by air. The forms in constant use
are made by the Eames, the American, the Westinghouse, and the
Beals companies. Nor can much be said here of the water-brake,
used to some extent on locomotives working heavy grades. It
consists of a simple arrangement of admitting a little hot water,
instead of steam, to the cylinders. The engine is reversed and the
cylinder-cocks are opened to the air. The cylinders then act as
air-pumps, and the retarding effect is due to the back pressure.
The use of the water is to prevent overheating of the parts.
[Illustration: Semaphore Signal with Indicators.
(One arm governs several tracks. The number of the track which is
clear is shown on the indicator disk.)]
If it is important to have efficient means of stopping trains,
it is scarcely less important to have timely information of the
need of stopping them. To give such information is the function
of signals, which, among safety appliances, must stand next after
brakes. Signals fall naturally into two great classes: Those which
protect points of danger and govern the movements of engines in
yards, and those which keep an interval of space between two trains
running on one track. For the protection of switches, crossings,
junctions, and the like, signals in immense variety have been used,
and, unfortunately, are still used; but in the last ten or fifteen
years the semaphore signal has become the general standard in the
United States, as it long has been in England. This consists of a
board, called the blade or arm, pivoted on the post, and back of
the pivot is a heavy casting which carries a colored glass lens,
either green or red. On the post is hung a lantern. The danger
position is with the blade horizontal. In this position the lens is
in front of the lamp, and the light shows red or green, as the case
may be. The safety position is with the blade hanging about sixty
degrees from the horizontal. In this position the light of the
lantern shows white. Red is the universal danger color, and green
the color of caution. Therefore, a semaphore signal at a point
of danger shows by day a blade painted red, with the end of the
blade cut square. At night it shows a red light. At a position some
distance from the point of actual danger, but where it is desirable
to warn an engine-runner that he is likely to find the danger
signal against him, a caution signal is placed. This is a semaphore
blade painted green, with the end notched in a V-shape, or, as it
is called, a fish-tail. At night this signal shows a green light.
There is nothing very remarkable about a piece of board arranged to
wag up and down on a pin stuck through a post, but it is wonderful
how much of good brains and good breath have been expended in
getting these boards to wag harmoniously, and in getting railroad
officers to understand that a plain board, having two possible
positions, is a better signal than any more complicated form.
[Illustration: Section of Saxby & Farmer Interlocking Machine.
(Showing two levers and locking mechanism. _A_ is normal, _B_ is
reversed.)]
The arrangement of a group of signals and switches in such a way
that their movements are made mutually dependent one upon the
other, and so that it is impossible to make these movements in
any but prearranged sequences, is called, in railroad vernacular,
"interlocking," and in this sense the word will be used here.
Interlocking has become a special art. The objects which it is
sought to accomplish by interlocking, and the admirable way in
which those objects are attained, may best be understood from an
actual example. For that purpose we shall take a double-track
junction completely equipped with signals, facing-point locks, and
derailing switches (p. 205).
A general view of an interlocking frame was given on page 171 of
this volume. Two levers from such a frame are here shown. The
normal position of the levers is forward, as lever _A_. When pulled
back, as lever _B_, the lever is said to be reversed.
Let it be supposed that a main-line train is to be passed eastward
in the direction of the arrow _B_. The first movement of the
signalman in the signal-tower would naturally be to lower signals
1 and 2. He attempts to pull over lever 1, but cannot move it,
and, in spite of any effort or ingenuity on his part, that signal
remains at danger. The reason is that lever 2 when normal locks
lever 1 normal. The logic of this will be at once apparent.
Clearing signal 1 is an indication to the engineer that the way
is clear, and that he may pass the junction at speed. So long as
this signal (which, it must be remembered, is a _caution_ signal)
stands at danger he knows that he may pass it, but must be ready to
stop before he reaches No. 2, the home-signal. Therefore No. 1 must
never be lowered till all is arranged for passing the junction at
speed. As the signalman cannot lower signal 1, he attempts to lower
signal 2. Again he finds that he cannot budge the lever. It is
locked by lever No. 3. This lever works a facing-point lock, which
must be described just at this point (p. 206).
[Illustration: Diagram of a Double-track Junction with Interlocked
Switches and Signals.
_A_ is the west-bound main line track; _B_, the east-bound; _C_ and
_D_ are the west-bound and east-bound branch-tracks. Nos. 1, 10,
and 12 are distant signals; Nos. 2, 9, and 11, home signals; Nos.
3, 6, and 8, facing-point locks; and Nos. 4, 5, and 7 are switches.
The levers which move all of these parts are placed side by side
in a frame in the signal-tower. It will be noticed that No. 7 is a
switch designed merely to derail an engine on track A. A similar
switch is provided on track _C_, and is worked by the same lever
which works junction switch No. 5. In the sketch all levers are
supposed to stand in their "normal" position, all signals are at
danger, and the switches are set for the main line. The switches
themselves are not locked in this position of the facing-point lock
levers.]
The front rod of the switch, that is, the rod which connects the
points of the two moving rails of the switch, is pierced with
two holes placed a distance apart just equal to the throw of the
switch. In front of these holes is a bolt which is worked by a
lever in the signal-tower. After the switch is set the lock-lever
is reversed and the bolt enters one of the holes, thus securely
locking the switch in position. There is one other interesting
feature of this facing-point lock. It has happened very often
that a switch has been thrown under a moving train, splitting the
train and derailing more or less of it. This class of accidents is
especially likely to happen when train movements are very frequent,
and may be prevented by the use of the "detector-bar." This is a
bar about forty feet long, placed alongside the rail, and carried
on swinging links, like those of a parallel ruler, in such a way
that any effort to move the bar lengthwise of the rail must raise
it above the top of the rail. This bar is moved by the same lever
which moves the locking-bolt. So long as there is a wheel on the
rail above the detector-bar it cannot be moved, therefore the
locking-bolt cannot be withdrawn, and the switch cannot be moved
until the train has passed completely off it.
[Illustration: Split Switches with Facing-point Locks and
Detector-bars.
(The rod on the right of the track is the mechanical connection to
the lever in the signal-tower by which the locks and detector-bars
are moved.)]
[Illustration: Derailing Switch.]
We left the signalman trying to lower signal No. 2; vainly, because
No. 3 lever was still normal and the switch unlocked (Diagram,
p. 205). Probably he would not have begun his operations in the
bungling way that has been supposed, but would have first reversed
lever 3. That locks the switch by the facing-point lock, and locks
also switch-lever 4 in the frame in the signal-tower and releases
lever 2. Then he reverses lever 2. That locks lever 3 and releases
lever 1. Then he reverses lever 1, which locks lever 2. Now the way
is made for a train to pass east on the main line, and the signals
are clear. The last signal could not have been lowered until the
chain of operations was complete; none of the levers can now be
moved until lever 1 is again put normal and signal 1 made to show
danger. There is one point of great danger in this particular
train-movement which has not been mentioned; that is, the crossing
of main-line east-bound track _B_ by the branch-line west-bound
track _C_. It will be noticed that with the levers normal,
derailing switch 5 is open, and it is impossible for a locomotive
to pass beyond it. Lever 5 is interlocked in the tower with lever
4 in such a way that, before 5 can be reversed to let a train pass
west from _C_, lever 4 must be reversed to trap any train on _B_
and turn it down the branch _D_. It must not be understood that the
use of "derailers" is universal. In fact, they are not recommended
by the best signal engineers, except in special conditions. In the
absence of derailer No. 5, signals 11 and 12 would be interlocked
with switch 4, so that, so long as that switch stands open for the
main line a clear signal cannot be given to a train coming west
on _C_. It will be noticed that signal 2 carries two semaphores
on one post. The upper one is for the main line and the lower one
for the branch. Both are operated by one lever, 2, and whether
reversing lever 2 lowers the main-line signal or the branch signal
depends on the position of the switch. The switch is made to pick
out its signal by an ingenious but very simple little arrangement,
called a selector, which is placed somewhere in the line of ground
connections.
It would be an interesting study, were there space, to follow
the possible and proper combinations of movements to pass trains
over the various tracks. It will be seen that, by concentrating
the levers which move switches and signals in one place and
interlocking them, it is made mechanically impossible for a
signalman to give a signal which would lead to a collision or a
derailment within the region under his control. The only danger
at such points is that an engineer may overrun the signals. This
description of the objects and the capacity of the system of
interlocking is no fancy sketch. The system has been in use for
many years, doing just what has been here described, and more. A
recent close estimate gave the number of interlocked levers now in
use in the United States as about eight thousand, and the number
is rapidly increasing. Recent official reports showed that in
Great Britain and Ireland there were thirty-eight thousand cases
in which a passenger line was connected with or crossed by another
line, siding, or cross-over. In eighty-nine per cent. of these
cases the levers operating the switches and protecting signals were
interlocked.
The example of interlocking which has been given is one of the
simplest; the principle is capable of almost indefinite expansion,
and any one lever may be made to lock any one or more levers among
hundreds in the same frame. The greatest number of levers assembled
in any one signal-tower in this country is one hundred and sixteen,
at the Grand Central Station in New York. In the London Bridge
tower there are two hundred and eighty levers. This is probably the
greatest number in any one tower in the world. All of these levers
may be more or less interlocked. The same principle is applied to
the locking of two levers at a single switch, and to the protection
of drawbridges and highway crossings.
The mechanism by which the interlocking is done is strong and
comparatively simple, but a detailed description of it seems out
of place here. Two levers from a Saxby & Farmer machine are shown
on page 204, with lever _A_ normal and _B_ reversed. The locking
mechanism is in front of the levers, and is actuated not by the
levers themselves, but by their catch-rods. It follows that it is
not the actual movement of a signal which prevents the movement of
other signals, or of switches, but it is the intention to move that
signal. This principle of "preliminary locking" is one of great
importance.
Switches and signals are often worked at such distances from the
tower that it is impossible for the operator to know whether or
not the movement contemplated has taken place. The British Board
of Trade does not permit switches to be worked more than 750 feet
away. In this country there is no limit, but probably 800 feet is
very rarely exceeded. Signals are worked in England up to 3,000 or
3,500 feet very commonly, and they are even worked a mile away,
but not satisfactorily. This is with direct mechanical connection,
by rod or wire, from the levers. It is obvious that a break in the
connections between the lever and the switch or signal might take
place, and the lever be pulled over, without having produced the
corresponding movement at the far end. The locking mechanism in the
tower would not be affected by such an accident, and consequently
conflicting signals might be given. Even this contingency is
provided against with almost perfect safety. If a signal connection
breaks, the signal is counter-weighted to go to danger. The worst
that can happen is to delay traffic. If a switch connection breaks,
the locking-bolt, in the latest form of facing-point lock, will not
enter the hole in the switch-rod, and consequently warning is given
in the tower that the switch has not moved. Electric annunciators
are often placed in the signal-tower, to show on a board before the
operator whether or not the movements of switches and signals have
taken place.
Considerable work must be done in the movement of each lever.
The ground connections must be put down with great care, as
nearly straight and level as may be, well drained, and protected
from ice and snow. All of these difficulties have been overcome
in a beautiful pneumatic interlocking apparatus which has been
introduced within the last two or three years. In this system
the motive power is compressed air. Near each switch is a small
cylinder, containing a piston which is attached directly to the
switch movement. Compressed air admitted to one side or the other
of this piston moves the switch one way or the other. But, as it
would take some time for the necessary quantity of air to flow
from the signal-tower to a distant switch, a small reservoir
is placed near the switch, and the air from this reservoir is
admitted to one end or the other of the switch cylinder according
to the position of a valve. For transmitting the motion from the
tower to the valve compressed air might be used, but, as air is
elastic, a quicker movement is got by using in the pipes some
liquid which does not readily freeze, and which, being practically
non-compressible, transmits an impulse given at one end almost
instantly to the other. The signals are worked in essentially the
same manner as the switches, except that the pneumatic valves are
moved by electricity. The tower apparatus of a pneumatic system in
the yard of the Pennsylvania Railroad at Pittsburg is shown in the
engraving opposite. In the front of the apparatus is seen a rank
of small handles, which can be turned from side to side with as
much ease as the keys of a piano can be depressed. Turning one of
these handles admits compressed air to the end of a pipe containing
liquid. Instantly the pressure is transmitted 500 or 1,000 feet
to the valve at the switch to be moved. The small levers are
interlocked perfectly, and in that particular perform the duties of
the ordinary machine. A model of the tracks controlled is placed
before the operator, showing the switches and signals, and when
a movement is made on the ground it is at once repeated back by
electricity and duplicated on the model. This beautiful system is
due to the same genius that gave us the perfected air-brake and the
triple valve, and is the greatest improvement that has been made in
interlocking in the last dozen years.
[Illustration: Interlocking Apparatus for Operating Switches and
Signals by Compressed Air, Pittsburg Yards, Pennsylvania Railroad.
(A model of the track is shown above the levers, on which the
movements of the switches and signals are electrically indicated
after they are completed.)]
[Illustration: Torpedo Placer.
(The torpedo is carried forward by the plunger and exploded by the
depression of the hammer shown near the rail.)]
If the reader has grasped the full significance of interlocking, he
understands that it makes it impossible to give a signal that would
lead to a collision or to a derailment at a misplaced switch. The
worst that a stupid, or drunken, or malicious signalman could do
would be to delay traffic, if the signals were obeyed. Here comes
in the failing case. The brake-power may be insufficient to stop a
train after a danger signal is given. That is a rare occurrence,
but may happen. The engineer may not see the danger signal because
of fog, or he may carelessly run past it. Provision against a
failure to see and to obey a signal may be made by placing on the
track a torpedo, which will explode with a loud report when struck
by a wheel. The use of hand-torpedoes in fogs, and for emergencies
in places unprovided with fixed signals, is very common. These are
little disks filled with a detonating powder, and provided with
tin straps that are bent down to clasp over the top of the rail.
A simple and very efficient torpedo machine, which has been used
for some years on the Manhattan Elevated and elsewhere, is here
shown. This machine has a magazine holding five torpedoes. It is
connected to a signal-lever in such a way that, when the signal is
put to danger, one torpedo is placed in a position to be exploded
by the first passing wheel. When the signal returns to the clear
position the torpedo, if unexploded, is withdrawn to the magazine.
If the torpedo is exploded another one takes its place at the next
movement of the signal-lever. One of these machines on the Elevated
Road moves about five thousand times every day. In such a case a
torpedo would soon be worn out if it was not exploded or frequently
changed. When this apparatus is in operation, an unmistakable
alarm is at once given to the engineer and to others if a danger
signal is passed. On the Manhattan Elevated lines an engineman who
overruns a danger signal and can show no good reason for it is
suspended for the first offence, and discharged for the second. The
torpedo makes it impossible for him to escape detection.
* * * * *
[Illustration: Old Signal Tower on the Philadelphia & Reading, at
Phœnixville.]
The second great class of signals comprises those which are
intended to keep fixed intervals of space between trains running
on the same track. These are block signals. The block system is
used on a few of the railroads of the United States which have
the heaviest and fastest traffic. Much the most common practice
in this country, however, is to run trains by time intervals, and
under the constant control of the train despatcher. In England the
block system is almost universal. About ninety per cent. of all the
passenger lines of that country are worked under the absolute block
system.