New York City Subway chaining

New York City Subway chaining is a method to precisely specify locations along the New York City Subway lines. It measures distances from a fixed point, called chaining zero, following the twists and turns of the railroad line, so that the distance described is understood to be the "railroad distance," not the distance by the most direct route ("as the crow flies"). It is an example of the chainage system, instead of the milepost or mileage system.

The New York City Subway system differs from other railroad chaining systems in that it uses the engineer's chain of 100 feet (30.48 m) rather than the surveyor's chain of 66 feet (20.1168 m).

Chaining zero is a fixed point from which the chaining is measured on a particular chaining line. A chaining number of, for example, 243 at a specific line location (called a chaining station) identifies that the location is the length of 243 100-foot chains (24,300 feet or 4.60 miles or 7.41 kilometres) from chaining zero, usually measured along the center line of the railroad.

Once chaining is established, it is rare but not unheard of to change the location of chaining zero or the route along which it is measured on a given line. There are several examples of chaining numbers that refer to a chaining zero location that no longer exists or along a physical line that no longer exists, because of abandonment or demolition. Notable among these are several existing chaining lines that originated near New York City Hall via the Brooklyn Bridge, discontinued since 1944. It is very rare but not impossible for a reroute to alter the accuracy of chaining numbers, if only slightly.

Exceptions exist to the principle that chaining numbers represent a railroad distance to the zero point. On the original IND chaining zero for the original system is a political rather than physical location, and there is no railroad at or near the zero point. Sometimes trackage (usually but not always short distances) is chained backwards from a tie point with another line.

The three divisions each had one separate Mileage zero before the 1958 opening of the Far Rockaway–Mott Avenue station. The zeros were changed after the Far Rockaway station's opening, and are now at the railroad southernmost points of each division. The IND division-wide zero is at the Far Rockaway station's bumper blocks, the IRT zero (except the IRT Flushing Line) is a northbound home signal at the north end of New Lots Avenue station, and the BMT zero is in the center of Coney Island–Stillwell Avenue station. Since the IRT Flushing Line is operationally separate from the rest of the system, it uses a different zero from the rest of the IRT. The IRT Flushing Line's Mileage zero is at 34th Street–Hudson Yards, and before the 7 Subway Extension opened in 2015, the zero was at Times Square.

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