RDSO Standard, New York City Subway

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The system also has automatic and manual key-by red lights. They involve the operation of an automatic stop with an automatic or manual release, then a procedure with caution, with preparations to stop in case of debris or other obstructions on the track.[8]

Speed control on the subway is ensured by "Time Signals".[9] A timer is started as soon as the train passes a certain point and will clear the signal ahead as soon as the predefined time elapsed; the minimum time is calculated from the speed limit and the distance between start of timer and signal.[9] "Time Signals" are distinguished into "Grade Timer" for speed supervision at grades, curves or in front of buffer stops, and in "Station Timer" to allow a following trains enter a station as another is leaving as long as they are going at a reduced speed.[9] There are two types of Grade Time signals. The first time, "two-shot timer," are generally used on down grades where the train must be under a set speed for a longer length of track. "One-shot timers," on the other time are found at sharp curves.[10]

Another addition to the system's transit signals are "Wheel Detectors." First introduced in 1996 at interlockings, they further enforce the speed in which a train is traveling through an interlocking, and they are only active when a switch is set to the divergent route.[10]

"Gap Filler Signals" are used at the 14th Street–Union Square and South Ferry Loop stations, where gap fillers extend out from the platforms to bridge the space between the platform and the car body and door at the curved stations. When the signals are red, the gap fillers are extended, and when the red light is no longer lit up, the gap fillers have been retracted, and the train operator can increase the speed of the train and leave the station.[10]

Aside from some parts of the original IRT system, the entire subway uses this signalling system. On the only line that still has the IRT-type signals, the IRT Dyre Avenue Line, the lights, from bottom to top, are yellow, red, and green. On the rest of the system, the lights from top to bottom, are green, yellow, and red.[8]

These signals work by preventing trains from entering a "block" occupied by another train. Typically, the blocks are 1,000 feet (300 m) long, although some highly used lines, such as the IRT Lexington Avenue Line, use shorter blocks. Insulators divide the track segments into blocks. The two traveling rails conduct an electric current, as they are connected to an electric current. If the circuit is closed and electricity can travel across the rails without interruption, the signal will light up as green, as it is unoccupied by a train. When a train enters the block, the metal wheels interrupt the current on the rails, and the signal turns red, marking the block as occupied. The train's maximum speed will depend on how many blocks are open in front of it. However, the signals do not register the trains' speed, nor do they register where in the block the train is located.[2] If a train passes a red signal, the train stop automatically engages and prevents the train from moving forward.[7]

Until 1970, when a train paused at a red block signal, the train operator was allowed to pass the red signal using a procedure called "keying by." The train operator would pause at the red signal, and then the operator would get out of the cab, descend to the trackside, and crank down the track trip with a key-like device. After a series of accidents, in which train operators keyed by and crashed into trains in front, the procedure was made against the rules, unless permission was granted by the train dispatchers.[11][12][13]

Types of interlocking signals[edit]

Interlockings are places where tracks are connected by switches and where routes may conflict. They are arranged in a specific way, with switches and signals that prevent conflicting movements once one route is set up. Interlocking signals are fixed signals within an interlocking, containing two separate red-yellow-green signal heads and often other indications.[12] A home signal is defined as an interlocking signal at the entrance to a route or block to control trains entering that route or block. These terms are used interchangeably, as most interlockings only have one controlled signal on each track.[1]

They are controlled by human operators in a signal tower near the switches, not by the trains themselves. A train operator must use a punch box, which is located besides the cab window in the station closest to the interlocking, to notify the switch operator of which track the train needs to go. The operator has a switchboard in their tower that allows them to change the switches.[7][1]

Interlocking signals also tell switch operators which way switches on the subway are set. The top part of an interlocking signal indicates the condition of the block ahead, while the lower part indicates the route selected. The following interlocking signals are used on the New York City Subway:

- [pic 10]

Proceed, switch set to straight

- [pic 11]

Proceed with caution, switch set to straight, next signal is currently red

- [pic 12]

Proceed with caution, switch set to diverge

- [pic 13]

Stop and stay

- [pic 14]

Call on (train has been given permission to pass red signal)

- [pic 15]

Entering timed block, switch set to straight, next signal is red onlydue to grade timing

- [pic 16]

Entering timed block, switch set to diverge

- [pic 17]

Timed block, timer has not yet run out, next block is timed as well as lunar aspect is indicated (in this example this signal would only clear to yellow over green)[14]

Until the DeKalb Avenue reconstruction on the BMT in 1958, there was a unique situation in which there was a three-way switch. As there were three possible options, a special blue signal was used.[12]

Another type of signals are dwarf signals, which are often used at switches to allow for occasional movements against the usual direction of train