Subway Signals: Time Signals

From nycsubway.org

Time signals are (block) signals that use timers which determine the speed of an approaching train as part of their decision as to whether to allow it to pass or not. Time signals on the New York system are of two varieties, Grade Time (GT) and Station Time (ST). The fundamental principle of both is the same: the speed of a moving object, in this case a train, can be determined by measuring how long it takes to traverse a fixed distance -- the longer the time, the slower the train. By starting a timer when a train enters the track section (block) in approach to a time signal, the signal can ensure that the train is going as slow as it requires by not clearing (showing a green or yellow aspect) until that timer has run its full time. If the train arrives at the signal before the time has expired (i.e., the train is going too fast), the signal will not be clear, and the train will be tripped, i.e., forced to stop immediately (see train stops).

Grade Time (GT) and Station Time (ST) are not really types or classes of signals, but rather, signal behaviors that can appear as a feature of automatic, approach or home signals, and can, and often do, occur simultaneously in the same signal. Thus, when we speak of GT signals, for example, we are using a shorthand for "signals equipped with Grade Time control" as we will describe.

All of the signal aspects illustrated below are now employed system-wide. While there may still be some older signals on all divisions with GT and/or ST controls absent these special aspects, their time is running out, as it were.

Grade Time (GT) Control

Grade Time (GT) control enforces an unconditional speed limit for trains on grades, hence the name, or curves, or in approach to bumpers, by the mechanism described above. A GT signal is normally red, and its stop is in the tripping position. The signal will not clear unless a train approaches it at the required speed. The speed is usually posted on an illuminated sign like the one at left. If several GT signals appear in sequence, as is very common, the sign will precede the first of them. Of course, like any other automatic, approach, or home signals, GT signals cannot clear unless the track ahead of them (their control length) is clear, and in the case of home signals and approach signals, that the necessary interlocking conditions have been met: GT and ST are additional requirements on signal clearing.

Pictured at left is a one-shot GT signal displaying the lunar white aspect. It is not clear; its train stop is in the tripping position. Its indication is that the track ahead of it is clear (vacant), and the only reason that the signal itself is not clear is that its GT timer has not run (to completion or perhaps at all). If a train approaches it at or below the posted speed, it and its stop will clear before the train reaches it. If there were a train beyond it (within its control length), the timer would make no difference: the signal would be red, and stay red as long as that train were there beyond it, and the lunar white light would not be lit. As the name indicates, one-shot GT signals give the train operator one "shot" to "get it right" - either approach at the correct speed, or be tripped at the signal. This aspect originated on the BMT division.

Pictured at left is not a two-shot GT signal, but an aspect always associated with them. This aspect, which is a clear aspect, indicates that the next signal (the one after this one) is a two-shot GT signal, which will simply look like a red signal at the time this aspect can be seen on the signal preceding it. What is more, it indicates that:

  1. The next signal, the two-shot GT signal, is at this time red.
  2. The only reason it is red is that its timer has not run; its control length is vacant, and interlocking constraints, if any, have been met.
  3. If the train approaches this signal slowly enough, the next signal will clear, and probably this signal will turn from yellow to green (and the S go out).
  4. Should the train operator fail to approach this signal slowly enough, the train will not be tripped, but will have a second try ("shot") at approaching the (next, the) two-shot GT signal, (hence the name) slowly enough, clearing it (and the signal beyond it, if that is also a two-shot GT at the same or higher speed limit, as well) if successful. Not one, but two timers are involved for each signal.

Two-shot GT signals promote smoother operation over long stretches than do one-shot GT's because the train operator can react more smoothly absent fear of being tripped by the dicier one-shot signals. When a train is progressing through a string of two-shot GT signals at the right speed and making all the "first tries", the operator (or railfan) will see the yellow-and-S aspect above change to green one signal at a time as approached, giving the false impression that signal with that aspect has the timer, when it is in fact the signal beyond it that is doing the timing. This, and the following, aspect originated on the IND division.

Pictured at left is a variant of the above aspect which is used when a home signal is a 2-shot GT signal; this aspect is displayed on the signal before that home signal, when the home signal is set for a diverging route, but the train operator cannot know that because it is at (double) red because it is a GT signal whose timer has not yet run out. If the train operator does not expect to take the diverging route, i.e., must contact the tower for a new route, he or she must stop the train rather than trying to clear the GT signal. Absent this aspect, the train operator would not be able to stop the train in time to request a new route if the GT home signal cleared to a indicate a route other than the one he or she expected, as the train passed the GT home signal at the required speed. Note that this not the aspect of a GT home signal, or the way that diverging route is indicated on a home signal.

"One-shot" GT is more common when not in runs of many, or on bumper or yard tracks or other low-speed areas. It is even possible for a signal to display one-shot and two-shot aspects at different times, for instance, when approached via different tracks (e.g., J2-234 at Myrtle Avenue on the BMT Broadway-Brooklyn line).

Station Time (ST) Control

Station time (ST) control is a signal feature that allows trains to "close in" on each other if they sufficiently reduce speed. It is normally used at stations to allow trains to slowly come closer to each other than would otherwise be allowed. This is desirable because a train stopping at a station while another train approaches of necessity reduces the distance between the two. Station time keeps trains moving.

An ST Signal is sort of like a part-time one-shot GT Signal. While a GT signal unconditionally requires trains to approach it slowly, and is not normally clear, an ST signal (unless it also a GT signal!, and, of course, interlocking constraints, where applicable, met) is normally clear, and requires trains to approach slowly only in a special circumstance. That circumstance is that only the tail of its control length is occupied.

For example, a signal might have a control length of 600 feet. That means that as long as there is any part of a train in the 600 feet beyond the signal, it will be red (at "stop"), and its train stop in the "tripping position": it keeps trains 600' apart. But if this signal is an ST signal, and the first (for example) 500' of its control length is clear (i.e., has no trains in it), and there is a train in only the last (farthest from the signal) 100', it will be willing to let the next train get within 500' of the first if it approaches the signal at 20 mph or less. In that case, it will display the aspect at left, which indicates, "approach this signal at 20 mph and it will clear." The aspect at left is uniquely associated with ST signals, and indicates their characteristic and unique "ST" state. It was introduced on all divisions in the 1960's, although ST controls existed well before then.

The start of ST control is indicated by fixed signs such as those at left. ST signals usually come in sequences, designed to allow trains to smoothly and safely close in on trains stopped in and leaving stations. Some older ST signals, especially on the IND Division, do not have the special ST aspect, although their number is dwindling. When ST control is added to interlocked signals (home and approach signals) very complex interactions and constraints ensue, which we will not discuss here. The interested reader is referred to the NXSYS, Signalling and Interlocking Simulator for further information on this.

Station Time and Grade Time Together

In more recent signalling, GT and ST control often appear in the same signal. This is a perfectly reasonable scenario for signals approaching and entering a station on a curve or grade. A little thought reveals that the ST speed limit must be slower than the GT speed limit, for while the ST speed limit must be obeyed for the signal to clear in the special circumstance described above, the GT limit must be obeyed for the signal to clear at all.

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