SIGNAL SYSTEM
Early in the development of the plans for the subway system in New
York City, it was foreseen that the efficiency of operation of a road
with so heavy a traffic as is being provided for would depend largely
upon the completeness of the block signaling and interlocking systems
adopted for spacing and directing trains. On account of the importance
of this consideration, not only for safety of passengers, but also for
conducting operation under exacting schedules, it was decided to
install the most complete and effective signaling system procurable.
The problem involved the prime consideration of:
Safety and reliability.
Greatest capacity of the lines consistent with the above.
Facility of operation under necessarily restricted yard and
track conditions.
In order to obtain the above desiderata it was decided to install a
complete automatic block signal system for the high-speed routes,
block protection for all obscure points on the low-speed routes, and
to operate all switches both for line movements and in yards by power
from central points. This necessarily involved the interconnection of
the block and switch movements at many locations and made the adoption
of the most flexible and compact appliances essential.
Of the various signal systems in use it was found that the one
promising entirely satisfactory results was the electro-pneumatic
block and interlocking system, by which power in any quantity could be
readily conducted in small pipes any distance and utilized in compact
apparatus in the most restricted spaces. The movements could be made
with the greatest promptness and certainty and interconnected for the
most complicated situations for safety. Moreover, all essential
details of the system had been worked out in years of practical
operation on important trunk lines of railway, so that its reliability
and efficiency were beyond question.
The application of such a system to the New York subway involved an
elaboration of detail not before attempted upon a railway line of
similar length, and the contract for its installation is believed to
be the largest single order ever given to a signal manufacturing
company.
In the application of an automatic block system to an electric railway
where the rails are used for the return circuit of the propulsion
current, it is necessary to modify the system as usually applied to a
steam railway and introduce a track circuit control that will not be
injuriously influenced by the propulsion current. This had been
successfully accomplished for moderately heavy electric railway
traffic in the Boston elevated installation, which was the first
electric railway to adopt a complete automatic block signal system
with track circuit control.
The New York subway operation, however, contemplated traffic of
unprecedented density and consequent magnitude of the electric
currents employed, and experience with existing track circuit control
systems led to the conclusion that some modification in apparatus was
essential to prevent occasional traffic delays.
The proposed operation contemplates a possible maximum of two tracks
loaded with local trains at one minute intervals, and two tracks with
eight car express trains at two minute intervals, the latter class of
trains requiring at times as much as 2,000 horse power for each train
in motion. It is readily seen, then, that combinations of trains in
motion may at certain times occur which will throw enormous demands
for power upon a given section of the road. The electricity conveying
this power flows back through the track rails to the power station and
in so doing is subject to a "drop" or loss in the rails which varies
in amount according to the power demands. This causes disturbances in
the signal-track circuit in proportion to the amount of "drop," and it
was believed that under the extreme condition above mentioned the
ordinary form of track circuit might prove unreliable and cause delay
to traffic. A solution of the difficulty was suggested, consisting in
the employment of a current in the signal track circuit which would
have such characteristic differences from that used to propel the
trains as would operate selectively upon an apparatus which would in
turn control the signal. Alternating current supplied this want on
account of its inductive properties, and was adopted, after a
demonstration of its practicability under similar conditions
elsewhere.
[Illustration: FRONT VIEW OF BLOCK SIGNAL POST, SHOWING LIGHTS,
INDICATORS AND TRACK STOP]
After a decision was reached as to the system to be employed, the
arrangement of the block sections was considered from the standpoint
of maximum safety and maximum traffic capacity, as it was realized
that the rapidly increasing traffic of Greater New York would almost
at once tax the capacity of the line to its utmost.
The usual method of installing automatic block signals in the United
States is to provide home and distant signals with the block sections
extending from home signal to home signal; that is, the block sections
end at the home signals and do not overlap each other. This is also
the arrangement of block sections where the telegraph block or
controlled manual systems are in use. The English block systems,
however, all employ overlaps. Without the overlap, a train in passing
from one block section to the other will clear the home signals for
the section in the rear, as soon as the rear of the train has passed
the home signal of the block in which it is moving. It is thus
possible for a train to stop within the block and within a few feet of
this home signal. If, then, a following train should for any reason
overrun this home signal, a collision would result. With the overlap
system, however, a train may stop at any point in a block section and
still have the home signal at a safe stopping distance in the rear of
the train.
Conservative signaling is all in favor of the overlap, on account of
the safety factor, in case the signal is accidentally overrun. Another
consideration was the use of automatic train stops. These stops are
placed at the home signals, and it is thus essential that a stopping
distance should be afforded in advance of the home signal to provide
for stopping the train to which the brake had been applied by the
automatic stop.
Ordinarily, the arrangement of overlap sections increases the length
of block sections by the length of the overlap, and as the length of
the section fixed the minimum spacing of trains, it was imperative to
make the blocks as short as consistent with safety, in order not to
cut down the carrying capacity of the railway. This led to a study of
the special problem presented by subway signaling and a development of
a blocking system upon lines which it is believed are distinctly in
advance of anything heretofore done in this direction.
[Illustration: REAR VIEW OF BLOCK SIGNAL POST, SHOWING TRANSFORMER AND
INSTRUMENT CASES WITH DOORS OPEN]
Block section lengths are governed by speed and interval between
trains. Overlap lengths are determined by the distance in which a
train can be stopped at a maximum speed. Usually the block section
length is the distance between signals, plus the overlap; but where
maximum traffic capacity is desired the block section length can be
reduced to the length of two overlaps, and this was the system adopted
for the Interborough. The three systems of blocking trains, with and
without overlaps, is shown diagramatically on page 143, where two
successive trains are shown at the minimum distances apart for
"clear" running for an assumed stopping distance of 800 feet. The
system adopted for the subway is shown in line "C," giving the least
headway of the three methods.
[Illustration: PNEUMATIC TRACK STOP, SHOWING STOP TRIGGER IN UPRIGHT
POSITION]
The length of the overlap was given very careful consideration by the
Interborough Rapid Transit Company, who instituted a series of tests
of braking power of trains; from these and others made by the
Pennsylvania Railroad Company, curves were computed so as to determine
the distance in which trains could be stopped at various rates of
speed on a level track, with corrections for rising and falling to
grades up to 2 per cent. Speed curves were then plotted for the trains
on the entire line, showing at each point the maximum possible speed,
with the gear ratio of the motors adopted. A joint consideration of
the speeds, braking efforts, and profile of the road were then used to
determine at each and every point on the line the minimum allowable
distance between trains, so that the train in the rear could be
stopped by the automatic application of the brakes before reaching a
train which might be standing at a signal in advance; in other words,
the length of the overlap section was determined by the local
conditions at each point.
In order to provide for adverse conditions the actual braking
distances was increased by 50 per cent.; for example, the braking
distance of a train moving 35 miles an hour is 465 feet, this would be
increased 50 per cent. and the overlap made not less than 697 feet.
With this length of overlap the home signals could be located 697 feet
apart, and the block section length would be double this or 1394 feet.
The average length of overlaps, as laid out, is about 800 feet, and
the length of block sections double this, or 1,600 feet.
[Illustration: VIEW UNDER CAR, SHOWING TRIGGER ON TRUCK IN POSITION TO
ENGAGE WITH TRACK STOP]
The protection provided by this unique arrangement of signals is
illustrated on page 143. Three positions of train are shown:
"A." MINIMUM distance between trains: The first train has
just passed the home signal, the second train is stopped by
the home signal in the rear; if this train had failed to stop
at this point, the automatic stop would have applied the air
brake and the train would have had the overlap distance in
which to stop before it could reach the rear of the train in
advance; therefore, under the worst conditions, no train can
get closer to the train in advance than the length of the
overlap, and this is always a safe stopping distance.
"B." CAUTION distance between train: The first train in same
position as in "A," the second train at the third home signal
in the rear; this signal can be passed under caution, and
this distance between trains is the caution distance, and is
always equal to the length of the block section, or two
overlaps.
"C." CLEAR distance between trains: First train in same
position as in "A," second train at the fourth home signal in
the rear; at this point both the home and distant signals are
clear, and the distance between the trains is now the clear
running distance; that is, when the trains are one block
section plus an overlap apart they can move under clear
signal, and this distance is used in determining the running
schedule. It will be noted in "C" that the first train has
the following protection: Home signals 1 and 2 in stop
position, together with the automatic stop at signal 2 in
position to stop a train, distant signal 1, 2, and 3 all at
caution, or, in other words, a train that has stopped is
always protected by two home signals in its rear, and by
three caution signals, in addition to this an automatic stop
placed at a safe stopping distance in the rear of the train.
[Illustration: ELECTRO-PNEUMATIC INTERLOCKING MACHINE ON STATION
PLATFORM]
[Illustration: SPECIAL INTERLOCKING SIGNAL CABIN SOUTH OF BROOKLYN
BRIDGE STATION]
[Sidenote: _Description
of Block
Signaling
System_]
The block signaling system as installed consists of automatic
overlapping system above described applied to the two express tracks
between City Hall and 96th Street, a distance of six and one-half
miles, or thirteen miles of track; and to the third track between 96th
and 145th Streets on the West Side branch, a distance of two and
one-half miles. This third track is placed between the two local
tracks, and will be used for express traffic in both directions,
trains moving toward the City Hall in the morning and in the opposite
direction at night; also the two tracks from 145th Street to Dyckman
Street, a distance of two and one-half miles, or five miles of track.
The total length of track protected by signals is twenty-four and
one-half miles.
The small amount of available space in the subway made it necessary to
design a special form of the signal itself. Clearances would not
permit of a "position" signal indication, and, further, a position
signal purely was not suitable for the lighting conditions of the
subway. A color signal was therefore adopted conforming to the adopted
rules of the American Railway Association. It consists of an iron case
fitted with two white lenses, the upper being the home signal and the
lower the distant. Suitable colored glasses are mounted in slides
which are operated by pneumatic cylinders placed in the base of the
case. Home and dwarf signals show a red light for the danger or "stop"
indication. Distant signals show a yellow light for the "caution"
indication. All signals show a green light for the "proceed" or clear
position. Signals in the subway are constantly lighted by two
electric lights placed back of each white lens, so that the lighting
will be at all times reliable.
On the elevated structure, semaphore signals of the usual type are
used. The signal lighting is supplied by a special alternating current
circuit independent of the power and general lighting circuits.
A train stop or automatic stop of the Kinsman system is used at all
block signals, and at many interlocking signals. This is a device for
automatically applying the air brakes to the train if it should pass a
signal in the stop position. This is an additional safeguard only to
be brought into action when the danger indication has for any reason
been disregarded, and insures the maintenance of the minimum distance
between trains as provided by the overlaps established.
Great care has been given to the design, construction, and
installation of the signal apparatus, so as to insure reliability of
operation under the most adverse conditions, and to provide for
accessibility to all the parts for convenience in maintenance. The
system for furnishing power to operate and control the signals
consists of the following:
Two 500-volt alternating current feed mains run the entire length of
the signal system. These mains are fed by seven direct-current
motor-driven generators operated in multiple located in the various
sub-power stations. Any four of these machines are sufficient to
supply the necessary current for operating the system. Across these
alternating mains are connected the primary coils of track
transformers located at each signal, the secondaries of which supply
current of about 10 volts to the rails of the track sections. Across
the rails at the opposite end of the section is connected the track
relay, the moving element of which operates a contact. This contact
controls a local direct-current circuit operating, by compressed air,
the signal and automatic train stop.
Direct current is furnished by two mains extending the length of the
system, which are fed by eight sets of 16-volt storage batteries in
duplicate. These batteries are located in the subway at the various
interlocking towers, and are charged by motor generators, one of which
is placed at each set of batteries. These motor generators are driven
by direct current from the third rail and deliver direct current of 25
volts.
The compressed air is supplied by six air compressors, one located at
each of the following sub-stations: Nos. 11, 12, 13, 14, 16, and 17.
Three of these are reserve compressors. They are motor-driven by
direct-current motors, taking current from the direct-current buss
bars at sub-stations at from 400 to 700 volts. The capacity of each
compressor is 230 cubic feet.
[Illustration: MAIN LINE, PIPING AND WIRING FOR BLOCK AND INTERLOCKING
SYSTEM, SHOWING JUNCTION BOX ON COLUMN]
The motor-driven air compressors are controlled by a governor which
responds to a variation of air pressure of five pounds or less. When
the pressure has reached a predetermined point the machine is stopped
and the supply of cooling water shut off. When the pressure has fallen
a given amount, the machine is started light, and when at full speed
the load is thrown on and the cooling water circulation reëstablished.
Oiling of cylinders and bearings is automatic, being supplied only
while the machines are running.
Two novel safety devices having to do especially with the signaling
may be here described. The first is an emergency train stop. It is
designed to place in the hands of station attendants, or others, the
emergency control of signals. The protection afforded is similar in
principle to the emergency brake handle found in all passenger cars,
but operates to warn all trains of an extraneous danger condition. It
has been shown in electric railroading that an accident to apparatus,
perhaps of slight moment, may cause an unreasoning panic, on account
of which passengers may wander on adjoining tracks in face of
approaching trains. To provide as perfectly as practicable for such
conditions, it has been arranged to loop the control of signals into
an emergency box set in a conspicuous position in each station
platform. The pushing of a button on this box, similar to that of the
fire-alarm signal, will set all signals immediately adjacent to
stations in the face of trains approaching, so that all traffic may be
stopped until the danger condition is removed.
The second safety appliance is the "section break" protection. This
consists of a special emergency signal placed in advance of each
separate section of the third rail; that is, at points where trains
move from a section fed by one sub-station to that fed by another.
Under such conditions the contact shoes of the train temporarily span
the break in the third rail. In case of a serious overload or ground
on one section, the train-wiring would momentarily act as a feeder for
the section, and thus possibly blow the train fuses and cause delay.
In order, therefore, to prevent trains passing into a dangerously
overloaded section, an overload relay has been installed at each
section break to set a "stop" signal in the face of an approaching
train, which holds the train until the abnormal condition is removed.
[Illustration: THREE METHODS OF BLOCK SIGNALING]
[Illustration: DIAGRAM OF OVERLAPPING BLOCK SIGNAL SYSTEM
ILLUSTRATING POSSIBLE POSITIONS OF TRAINS RUNNING UNDER SAME]
[Sidenote: _Interlocking
System_]
The to-and-fro movement of a dense traffic on a four-track railway
requires a large amount of switching, especially when each movement is
complicated by junctions of two or more lines. Practically every
problem of trunk line train movement, including two, three, and
four-track operation, had to be provided for in the switching plants
of the subway. Further, the problem was complicated by the restricted
clearances and vision attendant upon tunnel construction. It was
estimated that the utmost flexibility of operation should be provided
for, and also that every movement be certain, quick, and safe.
All of the above, which are referred to in the briefest terms only,
demanded that all switching movements should be made through the
medium of power-operated interlocking plants. These plants in the
subway portions of the line are in all cases electro-pneumatic, while
in the elevated portions of the line mechanical interlocking has been,
in some cases, provided.
A list of the separate plants installed will be interesting, and is
given below:
Location. Interlocking Working
Machines. Levers.
MAIN LINE.
City Hall, 3 32
Spring Street, 2 10
14th Street, 2 16
18th Street, 1 4
42d Street, 2 15
72d Street 2 15
96th Street 2 19
WEST SIDE BRANCH.
100th Street, 1 6
103d Street, 1 6
110th Street, 2 12
116th Street, 2 12
Manhattan Viaduct, 1 12
137th Street, 2 17
145th Street, 2 19
Dyckman Street, 1 12
216th Street, 1 14
EAST SIDE BRANCH.
135th Street, 2 6
Lenox Junction, 1 7
145th Street, 1 9
Lenox Avenue Yard, 1 35
Third and Westchester Avenue Junction, 1 13
St. Anna Avenue, 1 24
Freeman Street, 1 12
176th Street, 2 66
---- ----
Total, 37 393
The total number of signals, both block and interlocking, is as follows:
Home signals, 354
Dwarf signals, 150
Distant signals, 187
----
Total, 691
Total number of switches, 224
It will be noted that in the case of the City Hall Station three
separate plants are required, all of considerable size, and intended
for constant use for a multiplicity of movements. It is, perhaps,
unnecessary to state that all the mechanism of these important
interlocking plants is of the most substantial character and provided
with all the necessary safety appliances and means for rapidly setting
up the various combinations. The interlocking machines are housed in
steel concrete "towers," so that the operators may be properly
protected and isolated in the performance of their duties.