Most trains on the New York City Subway are manually operated. , the system currently uses automatic block signaling, with fixed wayside signals and automatic train stops. Many portions of the signaling system were installed between the 1930s and 1960s. Because of the age of the subway system, many replacement parts are unavailable from signaling suppliers and must be custom-built for the New York City Transit Authority, which operates the subway. Additionally, some subway lines have reached their train capacity limits and cannot operate extra trains in the current system.
There have been two different schemes of signaling in the system. The current scheme is used on all A Division and B Division lines, originally built to the Brooklyn–Manhattan Transit Corporation (BMT) and Independent Subway System (IND)'s specifications. An older system was previously used on all of the A Division, but with the conversion of the IRT Dyre Avenue Line signals to the B Division scheme in September 2017, this system is no longer in use. Retrieved April 30, 2018
As part of the modernization of the New York City Subway, the Metropolitan Transportation Authority (MTA) plans to upgrade and automate much of the system with communications-based train control (CBTC) technology, which will automatically start and stop trains. The CBTC system is mostly automated and uses a moving block system—which reduces headways between trains, increases train frequencies and capacities, and relays the trains' positions to a control room—rather than a fixed block system. The implementation of CBTC requires new rolling stock to be built for the subway routes using the technology, as only newer trains use CBTC.
Block signaling
The New York City Subway system has, for the most part, used block signaling since its 1904 opening. , the system consists of about 14,850 signal blocks, 3,538 mainline switches, 183 major track junctions, 10,104 automatic train stops, and 339,191 signal relays. Fixed-block signals are used on about 85% of the subway system . Trains used to be controlled by signal towers at interlockings, but this was eventually phased out in favor of master towers. Eventually, these master towers were replaced by a single rail control center: the New York City Transit Power Control Center in Midtown Manhattan.
These signals work by preventing trains from entering a "block" occupied by another train. Typically, the blocks are 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 form a track circuit, as they conduct electric current. If the track circuit is open and electricity cannot travel between the rails, the signal will light up as green, as it is unoccupied by a train. When a train enters the block, the metal wheels close the circuit 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. If a train passes a red signal, the train stop automatically engages and activates the brakes.
The New York City Subway generally distinguishes its current signals between automatic signals, which are solely controlled by train movements; approach signals, which can be forced to display a stop aspect by the interlocking tower; home signals, whose route is set by the interlocking tower; and additional signals (such as call-on, dwarf, marker, sign, repeater and time signals).
Common automatic and approach signals consist of one signal head showing one of the following signal aspects: stop (one red light); with special rules for call-on and timer signals clear, next signal at clear or caution (one green light) proceed with caution, be prepared to stop at next signal (one yellow light)
Where different directions are possible, the subway uses both speed and route signaling. The upper signal head indicates the speed while the lower signal head is used for routes, with the main route shown in green and the diverging route shown in yellow.
The old signals break down more easily, since some signals have outlasted their 50-year service life by up to 30 years, and signal problems accounted for 13% of all subway delays in 2016. Additionally, some subway services have reached their train capacity limits and cannot operate extra trains with the current Automatic Block Signaling system.
Types of block signals Standard block signals The following indications are used for blocks where there are no track divergences (i.e. junctions) in the block immediately ahead.
Older signals on the former IRT lines may use different indications, which are not shown. The lights on the older IRT signals, from top to bottom, were green, red, and yellow. The lights on current signals, from top to bottom, are green, yellow, and red.
Key-by signals The system also has automatic and manual "key-by" red signals where the conductor can insert a physical key into the wayside block signal, changing the signal indication from red to yellow. The signals involve the operation of an automatic stop with an automatic or manual release, then to proceed with caution, with preparations to stop in case of debris or other obstructions on the track.
Until 1970, when a train paused at a red block signal the train operator was allowed to pass the red signal by keying-by. That is, the train operator pauses at the red signal and then gets out of the cab, descends to the tracks' level, and cranks 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, including a crash north of the Hoyt–Schermerhorn Streets station in 1970, the procedure was banned unless permission was granted by the train dispatchers.
Time signals Speed control on the subway is ensured by "time signals". A timer, counting up, 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. "Time signals" are distinguished into "grade timers" for speed supervision at grades, curves or in front of buffer stops, and "station timers" to allow one train to enter a station just as another is leaving, as long as these trains are going at a reduced speed. There are two types of grade-time signals. The first type, "two-shot timers", are generally used on down grades where the train must be under a set speed for a longer length of track. They are so named because a train operator would have two chances, or "shots", to pass the signal within the posted speed limit. "One-shot timers", on the other hand, are found at sharp curves, and are named because the operator only has one chance to pass the signal within the speed limit.
Repeater signals "Repeater signals" are also used to maintain safe train operation around curves. These signals repeat the indication of a signal up ahead around a curve, and are located on the opposite side of the track from the signals they repeat.
Wheel detectors Another addition to the system's transit signals are "Wheel Detectors". These are sensors that can determine how fast a train is moving based on how quickly the axles of a given car are moving. First introduced in 1996 at interlockings, they further enforce the speed at which a train is traveling through an interlocking, and they are only active when a switch is set to the divergent route. Wheel detectors prevent train operators from running slowly at the beginning of a grade timing segment and then increasing the speed beyond the allowable limit without being tripped. When the indication is flashing, the train is too fast and is about to be tripped.
Gap filler signals "Gap filler signals" are used at the 14th Street–Union Square station on the IRT Lexington Avenue Line. They were formerly used at the Brooklyn Bridge–City Hall station on the IRT Lexington Avenue Line, the South Ferry Loop station on the IRT Broadway–Seventh Avenue Line, and the Times Square–42nd Street station on the 42nd Street Shuttle. These stations are part of the A Division, which consists of lines built by the Interborough Rapid Transit Company (IRT). At these stops, gap fillers extend out from the platforms to bridge the space between the platform and the car body and door at the curved stations. The signal consists of a single red lamp and a "GF" indicator underneath. When the signal is 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.
Types of interlocking signals Interlockings consist of two or more tracks that are connected by railroad switches; routes typically may conflict at these locations. 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 green-yellow-red signal heads and often other indications. 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. Most interlockings only have one controlled signal on each track. Some home signals contain a third, yellow aspect, on the bottom, and are known as "call-on" signals. These signals allow the train operator to press a lever near the signal to lower the trip arm, allowing the train to pass the signal at slow speed even if the signal is displaying a red light. Similar signals can be found in yards. The three aspects on these signals, when all displaying yellow, allow the train operator to go past the signal at a slow speed without stopping the train.
Interlocking signals 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 to. The switch operator has a switchboard in their tower that allows them to change the switches.
Interlocking signals also tell train 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:
Variants Until the DeKalb Avenue junction's reconstruction in 1958, there was a three-way switch at that junction, the only one in the system. A special blue signal was used for the third option.
Another signal type is a "dwarf signal", which is often used at switches to allow for occasional movements against the usual direction of train traffic. Controlled manually from a tower, they are not part of ordinary operations, and they generally do not include trip arms.
Modifications to signals