Although not themselves signals, stops, or "trippers", or "automatic train stops", as they are sometimes known, are a key component of the New York City subway's signal system. They are and have always been used everywhere in the system to force trains to stop if and when they attempt to illegally pass a red signal (one indicating "stop"). The stop is a T-shaped metal rod about a foot long, usually painted bright yellow, at track level, to the right side of the track on the IRT Division and the left on the BMT and IND Divisions. When the stop is raised by the signal system to the "tripping" position, it engages a "trip cock" on the wheel frame (truck) of a passing train, which cuts power to its motors and applies its brakes in a "full emergency" application, bringing it to a screeching halt, very possibly causing discomfort or minor injury to passengers, but stopping the train as rapidly as possible. That action is called tripping the train. Every car (not just the first car) is equipped with tripcocks.

Stops are an integral part of the signal system, and the key to its safety strategy. All signals except dwarf signals have stops. The stops are operated by a heavy mechanical spring and either an electric motor or a pneumatic valve (the original IRT was all pneumatic in this regard) - if electric power or air pressure is deenergized, or fails, the stop is raised to the "tripping" position by the spring. The signal system, therefore, drives the stop (forces it down) when conditions are safe, not "raises" it when conditions are unsafe (this exemplifies the general "fail-safe" design of the signal system.)

Although the entire purpose of stops is to prevent trains from passing red signals, the stop associated with a properly-functioning signal will not necessarily be in the tripping position when the signal is red (although the signal always will be red if the stop is up). Here are several cases where the signal is red and the stop is clear (or driven):

1. Although a signal becomes red when the front of the train passes it, its stop must not come up until the end of the train passes it, or it will trip the very train that is passing it! On some other systems (e.g., the London Underground), this is not so -- only the tripcock of the first car is active and in a position to engage the trip; but in New York, all tripcocks are always active.
2. When a track has signals in both directions (e.g., the express track on three-track lines such as the Pelham Bay line (#6), or almost any section of track at an interlocking), when motion is cleared in one direction, the stops of the signals governing motion in the opposite direction will be cleared, even though their corresponding signals are red. This is to prevent off-side tripping, that is, the inadvertent tripping of trains by the stops of signals being passed in the reverse direction. It is doubly important that stops so driven come up as the rear of the train passes them, otherwise their function, to prevent motion in the opposite direction, is not served! Clearly, other constraints are imposed via interlocking to keep trains in the "wrong direction" from proceeding in.
3. It is possible, when signal repair work is being done, or in other circumstances where appropriate orders have been given, for authorized personnel to intentionally disable the stop (but the signal system knows that this has been done).
4. Automatic key-by. For an automatic or approach signal (not on the unresignalled IRT) indicating "stop", it is possible for the train to creep up very, very slowly, 1 or 2 mph, such that the front wheel of the train passes the track joint electrically separating signal blocks from each other, with still critical distance to spare between the stop and the train's tripcock. This maneuver, automatic key-by (AK), will drive the stop and hold it clear, and the train can proceed beyond the red signal prepared to stop within vision. Since the mid 1970's (when exactly?), this has not been permitted without special orders, on account of an accident resulting from abuse of "AK".
5. Manual key-by. Some older signals on the IRT (and formerly the BMT) offer manual key-by, by which automatic and approach signals at "stop" can be passed by the train operator stopping the train and operating a key (hence the name) or key-by button physically on the signal or on a box attached to it, which drives the stop.
6. Call-on. For a home signal indicating "stop and stay", it is possible under certain circumstances for the tower operator and train operator to cooperate to clear the stop and allow the train past the signal. This is called a call-on, and requires that a train is directly in front of the signal, the signal showing red over red, all the switches in the route lined up correctly, and the tower operator having operated a special call-on button associated with the home signal. When these conditions obtain, a third, special yellow light (the call on aspect, shown at right) appears under the signal, and the train operator, seeing this, presses and holds the signal's key-by button, with which each home signal is supplied, until the stop drives and stays down (accepting the call-on). The operator may then proceed past the home signal at very low speed, into possibly-occupied track, prepared to stop within vision.
   
   Note that although a call-on is usually given when the signal's control length (the track ahead of the signal) is occupied, the tower operator can force a call-on when the control length is vacant by operating the call-on button before the route is complete. But for this, the normal "high signal" (fully clear) aspect would appear as usual instead of the desired call-on aspect. This is sometimes done when the track circuits at an interlocking are operating erratically. See Interlocking.
7. Reverse running. On most trackage signalled for a single direction, a train running backwards (which is only permitted under the rarest of circumstances) will drive down the stops of automatic and approach signals as it passes them backwards. It is the responsibility of interlocking to prevent trains from entering trackage in inappropriate directions in the first place.

As anyone who has carefully watched the trains and tracks knows, in most signalling in the New York Subway, the stops of automatic and approach signals do not come up (assume the "tripping" position) until the end of the train passes the next signal: this confers the advantage that the occupancy of the track section immediately beyond the signal can be used to drive the stop down and implement automatic key-by, clearing for the reverse running scenario, and preventing trains from tripping themselves all in one fell swoop. On the other hand, it mandates that signal control be overlapped such that there is always more than one red signal behind any train.

(Important) The central principle of signal placement and engineering is that the control lengths of the signals be so designed such that when a train running at the highest possible speed under the worst weather conditions, with any semblance of functional brakes, is tripped, it will come to a halt before encountering the other train or obstruction that is causing the signal to be red.

All signals that have a stop associated with them have a built-in check to ensure that they cannot be clear unless the stop is actually clear; if the stop is broken or frozen or jammed in the tripping position, the signal will not clear: the signal has to first want to clear, then the stop clears, then the signal clears. What is more, each signal also has a stop cycle check, to ensure that when it is ready to clear, the stop really is now in the tripping position, i.e., has the capability to trip when the signal is not clear. This feature verifies complete and correct operation before a signal can clear. Stops and signals work hand-in-hand.

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Descriptions and graphics on this page (unless otherwise noted) are Copyright © 1997-2002 Bernard S. Greenberg (contact).