INTERLOCKING.
In their management of switches, especially at points of railroad
convergence where a heavy traffic is concentrated and the passage
of trains or movement of cars and locomotives is unceasing,
the English are immeasurably in advance of the Americans; and,
indeed, of all other people. In fact, in this respect the American
managers have shown themselves slow to learn, and have evinced an
indisposition to adopt labor-saving appliances which, considering
their usual quickness of discernment in that regard, is at first
sight inexplicable. Having always been accustomed to the old and
simple methods, just so long as they can through those methods
handle their traffic with a bearable degree of inconvenience and
expense, they will continue to do so. That their present method is
most extravagant, just as extravagant as it would be to rent two
houses or to run two steam engines where one, if properly used,
could be made to suffice, admits of demonstration;--but the waste is
not on the surface, and the necessity for economy is not imperative.
The difference of conditions and the difference in results may be
made very obvious by a comparison. Take, for instance, London and
Boston--the Cannon street station in the one and the Beach street
station in the other. The concentration of traffic at London is so
great that it becomes necessary to utilize every foot of ground
devoted to railroad purposes to the utmost possible extent. Not
only must it be packed with tracks, but those tracks must never be
idle. The incessant train movement at Cannon street has already
been referred to as probably the most extraordinary and confusing
spectacle in the whole wide circle of railroad wonders. The result
is that in some way, at this one station and under this single roof,
more trains must daily be made to enter and leave than enter and
leave, not only the Beach street station, but all the eight railroad
stations in Boston combined.[15]
[15] "It has been estimated that an average of 50,000 persons were,
in 1869, daily brought into Boston and carried from it, on three
hundred and eighty-five trains, while the South Eastern railway of
London received and despatched in 1870, on an average, six hundred
and fifty trains a day, between 6 A.M. and 12 P.M. carrying from
35,000 to 40,000 persons, and this too without the occurrence of a
single train accident during the year. On one single exceptional
day eleven hundred and eleven trains, carrying 145,000 persons, are
said to have entered and left this station in the space of eighteen
hours."--_Third Annual Report, [1872] of Massachusetts Railroad
Commissioners, p. 141._
The passenger movement over the roads terminating in Boston was
probably as heavy on June 17, 1875, as during any twenty-four hours
in their history. It was returned at 280,000 persons carried in
641 trains. About twice the passenger movement of the "exceptional
day" referred to, carried in something more than half the number of
trains, entering and leaving eight stations instead of one.
During eighteen successive hours trains have been made to enter and
leave this station at the rate of more than one in each minute. It
contains four platforms and seven tracks, the longest of which is
720 feet. As compared with the largest station in Boston (the Boston
& Providence), it has the same number of platforms and an aggregate
of 1,500 (three-fifths) more feet of track under cover; it daily
accommodates about nine times as many trains and four times as many
passengers. Of it Barry, in his treatise on Railway Appliances (p.
197), says: "The platform area at this station is probably minimised
but, the station accommodates efficiently a very large mixed traffic
of long and short journey trains, amounting at times to as many as
400 trains in and 400 trains out in a working day.[16]"
[16] The Grand Central Depot on 42d Street in New York City, has
nearly twice the amount of track room under cover of the Cannon
street station. The daily train movement of the latter would be
precisely paralleled in New York, though not equalled in amount, if
the 42d street station were at Trinity church, and, in addition to
the trains which now enter and leave it, all the city trains of the
Elevated road were also provided for there.
The American system is, therefore, one of great waste; for, being
conducted in the way it is--that is with stations and tracks
utilized to but a fractional part of their utmost capacity--it
requires a large number of stations and tracks and the services of
many employés. Indeed it is safe to say that, judged by the London
standard, not more than two of the eight stations in Boston are at
this time utilized to above a quarter part of their full working
capacity; and the same is probably true of all other American
cities. Both employés and the travelling public are accustomed to a
slow movement and abundance of room; land is comparatively cheap,
and the pressure of concentration has only just begun to make itself
felt. Accordingly any person, who cares to pass an hour during the
busy time of day in front of an American city station, cannot but
be struck, while watching the constant movement, with the primitive
way in which it is conducted. Here are a multiplicity of tracks all
connected with each other, and cars and locomotives are being passed
from one to another from morning to night. A constant shifting
of switches is going on, and the little shunting engines never
stand still. The switches, however, as a rule, are unprovided with
signals, except of the crudest description; they have no connection
with each other, and during thirty years no change has been made
in the method in which they are worked. When one of them has to be
shifted, a man goes to it and shifts it. To facilitate the process,
the monitor shunting engines are provided with a foot-board in front
and behind, just above the track, upon which the yard hands jump,
and are carried about from switch to switch, thus saving the time
they would occupy if they had to walk. A simpler arrangement could
not be imagined; anyone could devise it. The only wonder is that
even a considerable traffic can be conducted safely in reliance upon
it.
Turning from Beach to Cannon street, it is apparent that the
train movement which has there to be accommodated would fall into
inextricable confusion if it was attempted to manage it in the way
which has been described. The number of trains is so great and
the movement so rapid and intricate, that not even a regiment of
employés stationed here and there at the signals and switches could
keep things in motion. From time to time they would block, and then
the whole vast machine would be brought to a standstill until order
could be re-established. The difficulty is overcome in a very simple
way, by means of an equally simple apparatus. The control over
the numerous switches and corresponding signals, instead of being
divided up among many men stationed at many points, is concentrated
in the hands of two men occupying a single gallery, which is
elevated across the tracks in front of the station and commanding
the approaches to it, much as the pilot-house of an American steamer
commands a view of the course before it. From this gallery, by means
of what is known as the interlocking system, every switch and signal
in the yard below is moved; and to such a point of perfection has
the apparatus been carried, that any disaster from the misplacement
of a switch or the display of a wrong signal is rendered impossible.
Of this Cannon street apparatus Barry says, "there are here nearly
seventy point and signal levers concentrated in one signal house;
the number of combinations which would be possible if all the
signal and point levers were not interlocked can be expressed only
by millions. Of these only 808 combinations are safe, and by the
interlocking apparatus these 808 combinations are rendered possible,
and all the others impossible."[17]
[17] _Railway Appliances_, p. 113.
It is not proposed to enter at any length into the mechanical
details of this appliance, which, however, must be considered as one
of the three or four great inventions which have marked epochs in
the history of railroad traffic.[18] As, however, it is but little
known in America, and will inevitably within the next few years find
here the widest field for its increased use, a slight sketch of its
gradual development and of its leading mechanical features may not
be out of place. Prior to the year 1846 the switches and signals
on the English roads were worked in the same way that they are now
commonly worked in this country. As a train drew near to a junction,
for instance, the switchman stationed there made the proper track
connection and then displayed the signal which indicated what tracks
were opened and what closed, and which line had the right of way;
and the engine-drivers acted accordingly. As the number of trains
increased and the movement at the junctions became more complicated,
the danger of the wrong switches being thrown or the wrong signals
displayed, increased also. Mistakes from time to time would happen,
even when only the most careful and experienced men were employed;
and mistakes in these matters led to serious consequences. It,
therefore, became the practice, instead of having the switch or
signal lever at the point where the switch or signal itself was, as
is still almost universally the case in this country, to connect
them by rods or wires with their levers, which were concentrated
at some convenient point for working, and placed under the control
of one man instead of several. So far as it went this change was
an improvement, but no provision yet existed against the danger of
mistake in throwing switches and displaying signals. The blunder of
first making one combination of tracks and then showing the signal
for another was less liable to happen after the concentration of
the levers under one hand than before, but it still might happen at
any time, and certainly would happen at some time. If all danger of
accident from human fallibility was ever to be eliminated a far more
complicated mechanical apparatus must be devised. In response to
this need the system of interlocking was gradually developed, though
not until about the year 1856 was it brought to any considerable
degree of perfection. The whole object of this system is to
render it impossible for a switchman, whether because he is weary
or agitated or actually malicious or only inexperienced, to give
contrary signals, or to break his line in one way and to give the
signal for its being broken in another way. To bring this about the
levers are concentrated in a cabin or gallery, and placed side by
side in a frame, their lower ends connecting with the switch-points
and signals by means of rods and wires. Beneath this frame are one
or more long bars, extending its entire length under it and parallel
with it. These are called locking bars; for, being moved to the
right or left by the action of the levers they hold these levers in
certain designated positions, nor do they permit them to occupy any
other. In this way what is termed the interlocking is effected. The
apparatus, though complicated, is simplicity itself compared with
a clock or a locomotive. The complication, also, such as it is,
arises from the fact that each situation is a problem by itself, and
as such has to be studied out and provided for separately. This,
however, is a difficulty affecting the manufacturer rather than the
operator. To the latter the apparatus presents no difficulty which
a fairly intelligent mechanic cannot easily master; while for the
former the highly complicated nature of the problem may, perhaps,
best be inferred from the example given by Mr. Barry, the simplest
that can offer, that of an ordinary junction where a double-track
branch-road connects with its double-track main line. There would
in this case be of necessity two switch levers and four signal
levers, which would admit of sixty-four possible combinations. "The
signal might be arranged in any of sixteen ways, and the points
might occupy any of four positions, irrespective of the position
of the signals. Of the sixty-four combinations thus possible
only thirteen are safe, and the rest are such as might lure an
engine-driver into danger."
[18] A sufficiently popular description of this apparatus also,
illustrated by cuts, will be found in Barry's excellent little
treatise on _Railway Appliances_, already referred to, published by
Longmans & Co. as one of their series of text-books of science.
Originally the locking bar was worked through the direct action of
certain locks, as they were called, between which the levers when
moved played to and fro. These locks were mere bars or plates of
iron, some with inclined sides, and others with sides indented or
notched. At one end they were secured on a pivot to a fixed bar
opposite to and parallel with the movable locking bar, while their
other ends were made fast to the locking bar; whence it necessarily
followed that, as certain of the levers were pushed to and fro
between them, the action of these levers on the inclined sides of
the locks could by a skilful combination be made to throw other
levers into the notches and indentations of other locks, thus
securing them in certain positions, and making it impossible for
them to be in any other positions.
The apparatus which has been described, though a great improvement
on anything which had preceded it, was still but a clumsy affair,
and naturally the friction of the levers on the locks was so great
that they soon became worn, and when worn they could not be relied
upon to move the switch-points with the necessary accuracy. The new
appliance of safety had, therefore, as is often the case, introduced
a new and very considerable danger of its own. The signals and
switches, it was true, could no longer disagree, but the points
themselves were sometimes not properly set, or, owing to the great
exertion required to work it, the interlocking gear was strained.
This difficulty resulted in the next and last improvement, which
was a genuine triumph of mechanical ingenuity. To insure the proper
length of stroke being made in moving the lever--that is to make
it certain in each case that the switch points were brought into
exactly the proper position--two notches were provided in the slot,
or quadrant, as it is called, in which the lever moved, and, when
it was thrown squarely home, and not until then, a spring catch
caught in one or other of these notches. This spring was worked by
a clasp at the handle of the lever, and the whole was called the
spring catch-rod. By a singularly ingenious contrivance, the process
of interlocking was transferred from the action of the levers and
the keys to these spring catch-rods, which were made to work upon
each other, and thus to become the medium through which the whole
process is effected. The result of this improvement was that, as
the switchman cannot move any lever until the spring-catch rod is
fastened, except for a particular movement, he cannot, do what he
will, even begin any other movement than that one, as the levers
cannot be started. On the other hand, it may be said that, by means
of this improvement, the mere "intention of the signal-man to move
any lever, expressed by his grasping the lever and so raising the
spring catch-rod, independently of his putting his intention in
force, actuates all the necessary locking.[19]"
[19] In regard to the interlocking system as then in use in England,
Captain Tyler in his report as head of the railway inspecting
department of the Board of Trade, used the following language in
his report on the accidents during 1870. "When the apparatus is
properly constructed and efficiently maintained, the signalman
cannot make a mistake in the working of his points and signals which
shall lead to accident or collision, except only by first lowering
his signal and switching his train forward, then putting up his
signal again as it approaches, and altering the points as the driver
comes up to, or while he is passing over them. Such a mistake was
actually made in one of the cases above quoted. It is, of course,
impossible to provide completely for cases of this description; but
the locking apparatus, as now applied, is already of enormous value
in preventing accidents; and it will have a still greater effect
on the general safety of railway travelling as it becomes more
extensively applied on the older lines. Without it, a signalman in
constantly working points and signals is almost certain sooner or
later to make a mistake, and to cause an accident of a more or less
serious character; and it is inexcusable in any railway company to
allow its mail or express trains to run at high speed through facing
points which are not interlocked efficiently with the signals, by
which alone the engine-drivers in approaching them can be guided.
There is however, very much yet to be effected in different parts of
the country in this respect. And it is worth while to record here,
in illustration of the difficulties that are sometimes met with by
the inspecting officers, that the Midland Railway Company formally
protested in June, 1866, against being compelled to apply such
apparatus before receiving sanction for the opening of new lines
of railway. They stated that in complying with the requirements in
this respect of the Board of Trade, they '_were acting in direct
opposition to their own convictions, and they must, so far as lay in
their power, decline the responsibility of the locking system_.'"
To still further perfect the appliance a simple mechanism has
since 1870 been attached to the rod actuating the switch-bolt,
which prevents the signal-man from shifting the switch under a
passing train in the manner suggested by Captain Tyler in the above
extract. In fact it is no exaggeration to say that the interlocking
system has now been so studied, and every possible contingency so
thoroughly provided for, that in using it accidents can only occur
through a wilful intention to bring them about.
In spite of any theoretical or fanciful objections which may be
urged against it, this appliance will be found an indispensable
adjunct to any really heavy junction or terminal train movement. For
the elevated railroads of New York, for instance, its early adoption
proved a necessity. As for questions of temperature, climate,
etc., as affecting the long connecting rods and wires which are an
essential part of the system, objections based upon them are purely
imaginary. Difficulties from this source were long since met and
overcome by very simple compensating arrangements, and in practice
occasion no inconvenience. That rods may break, and that wires are
at all times liable to get out of gear, every one knows; and yet
this fact is urged as a novel objection to each new mechanical
improvement. That a broken or disordered apparatus will always
occasion a serious disturbance to any heavy train movement, may also
be admitted. The fact none the less remains that in practice, and
daily subjected through long periods of time to incomparably the
heaviest train movement known to railroad experience, the rods of
the interlocking apparatus do not break, nor do its wires get out
of gear; while by means of it, and of it alone, this train movement
goes unceasingly on never knowing any serious disturbance.[20]
[20] "As an instance of the possibility of preventing the mistakes
so often made by signal men with conflicting signals or with facing
points I have shown the traffic for a single day, and at certain
hours of that day, at the Cannon Street station of the South Eastern
Railway, already referred to as one of the _no-accident_ lines of
the year. The traffic of that station, with trains continually
crossing one another, by daylight and in darkness, in fog or in
sunshine, amounts to more than 130 trains in three hours in the
morning, and a similar number in the evening; and, altogether, to
652 trains, conveying more than 35,000 passengers in the day as
a winter, or 40,000 passengers a day as a summer average. It is
probably not too much to say, that without the signal and point
arrangements which have there been supplied, and the system of
interlocking which has there been so carefully carried out, the
signalmen could not carry on their duties _for one hour without
accident_." _Captain Tyler's report on accidents for 1870, p. 35._
It is not, however, alone in connection with terminal stations and
junctions that the interlocking apparatus is of value. It is also
the scientific substitute for the law or regulation compelling
trains to stop as a measure of precaution when they approach
grade-crossings or draw-bridges. It is difficult indeed to pass from
the consideration of this fine result of science and to speak with
patience of the existing American substitute for it. If the former
is a feature in the block system, the latter is a signal example of
the block-head system. As a device to avoid danger it is a standing
disgrace to American ingenuity; and, fortunately, as stopping is
compatible only with a very light traffic, so soon as the passage
of trains becomes incessant a substitute for it has got to be
devised. In this country, as in England, that substitute will be
found in the interlocking apparatus. By means of it the draw-bridge,
for instance, can be so connected with the danger signals--which
may, if desired, be gates closing across the railroad tracks--that
the one cannot be opened except by closing the other. This is the
method adopted in Great Britain not only at draws in bridges, but
frequently also in the case of gates at level road crossings. It
has already been noticed that in Great Britain accidents at draws
in bridges seem to be unknown. Certainly not one has been reported
during the last nine years. The security afforded in this case
by interlocking would, indeed, seem to be absolute; as, if the
apparatus is out of order, either the gates or the bridge would be
closed, and could not be opened until it was repaired. So also as
respects the grade-crossing of one railroad by another. Bringing
all trains to a complete stop when approaching these crossings
is a precaution quite generally observed in America, either as a
matter of statute law or running regulation; and yet during the six
years 1873-8 no less than 104 collisions were reported at these
crossings. In Great Britain during the nine years 1870-8 but nine
cases of accidents of this description were reported, and in both
the years 1877 and 1878 under the head of "Accidents or Collisions
on Level Crossings of Railways," the chief inspector of the Board
of Trade tersely stated that,--"No accident was inquired into under
this head.[21]" The interlocking system there affords the most
perfect protection which can be devised against a most dangerous
practice in railroad construction to which Americans are almost
recklessly addicted. It is, also, matter of daily experience that
the interlocking system does afford a perfect practical safeguard
in this case. Every junction of a branch with a double track
road involves a grade-crossing, and a grade-crossing of the most
dangerous character. On the Metropolitan Elevated railroad of New
York, at 53d street, there is one of these junctions, where, all
day long, trains are crossing at grade at the rate of some twenty
miles an hour. These trains never stop, except when signalled so
to do. The interlocking apparatus, however, makes it impossible
that one track should be open except when the other is closed. An
accident, therefore, can happen only through the wilful carelessness
of the engineer in charge of a train;--and in the face of wilful
carelessness laws are of no more avail than signals. If a man in
control of a locomotive wishes to bring on a collision he can always
do it. Unless he wishes to, however, the interlocking apparatus
not only can prevent him from so doing, but as a matter of fact
always does. The same rule which holds good at junctions would hold
good at level crossings. There is no essential difference between
the two. By means of the interlocking apparatus the crossing can
be so blocked at any desired distance from it in such a way that
when one track is open the other must be closed;--unless, indeed,
the apparatus is out of order, and then both would be closed.
The precaution in this case, also, is absolute. Unlike the rule
as to stopping, it does not depend on the caution or judgment of
individuals;--there are the signals and the obstructions, and
if they are not displayed on one road they are on the other. So
superior is this apparatus in every respect--as regards safety as
well as convenience--to the precaution of coming to a stop, that, as
an inducement to introduce an almost perfect scientific appliance,
it would be very desirable that states like Massachusetts and
Connecticut compelling the stop, should except from the operation
of the law all draw-bridges or grade-crossings at which suitable
interlocking apparatus is provided. Surely it is not unreasonable
that in this case science should have a chance to assert itself.
[21] "As affecting the safe working of railways, the level crossing
of one railway by another is a matter of very serious import.
Even when signalled on the most approved principles, they are a
source of danger, and, if possible, should always be avoided. At
junctions of branch or other railways the practice has been adopted
by some companies in special cases, to carry the off line under or
over the main line by a bridge. This course should generally be
adopted in the case of railways on which the traffic is large, and
more expressly where express and fast trains are run." _Report on
Accidents on Railways of the United Kingdom during 1877, p. 35._
In any event, however, the general introduction of the interlocking
apparatus into the American railroad system may be regarded as a
mere question of the value of land and concentration of traffic.
So long as every road terminating in our larger cities indulges,
at whatever unnecessary cost to its stockholders, in independent
station buildings far removed from business centres, the train
movement can most economically be conducted as it now is. The
expense of the interlocking apparatus is avoided by the very simple
process of incurring the many fold heavier expense of several
station buildings and vast disconnected station grounds. If,
however, in the city of Boston, for instance, the time should come
when the financial and engineering audacity of the great English
companies shall be imitated,--when some leading railroad company
shall fix its central passenger station on Tremont street opposite
the head of Court street, just as in London the South Eastern
established itself on Cannon street, and then this company carrying
its road from Pemberton Square by a tunnel under Beacon Hill and the
State-house should at the crossing of the Charles radiate out so as
to afford all other roads an access for their trains to the same
terminal point, thus concentrating there the whole daily movement of
that busy population which makes of Boston its daily counting-room
and market-place,--then, when this is attempted, the time will have
come for utilizing to its utmost capacity every available inch of
space to render possible the incessant passage of trains. Then also
will it at last be realized that it is far cheaper to use a costly
and intricate apparatus which enables two companies to be run into
one convenient station, than it is to build a separate station, even
at an inconvenient point, to accommodate each company.