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").

The New York City Subway system differs from other railroad chaining systems in that it uses the engineer's chain of rather than the surveyor's chain of .

Terminology

Chaining zero

''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 () 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 occasionally possible for a reroute to alter the accuracy of chaining numbers 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.

Chaining lines

''Chaining lines'' are routes on physical railroad lines that are usually described by one or two letters for the purpose of identifying locations on those lines.

''Chaining lines'' are ''not necessarily'' the same as the physical lines they run on. One physical line may have several chaining letters, and one chaining line may cover several physical lines.

The letters assigned to a chaining line have nothing to do with the letters displayed on trains, public maps and timetables. These letters are ''subway service'' letters. ''See:'' New York City Subway nomenclature and List of New York City Subway services.

For example, the BMT A chaining line begins at BMT South chaining zero north of 57th Street–Seventh Avenue on the BMT Broadway Line, but is interrupted north of the Canal Street stations, where the express tracks becomes BMT H for the trip over the Manhattan Bridge south side tracks and the local tracks become BMT B for Lower Manhattan and the Montague Street Tunnel. The BMT A line begins again in the middle of the Manhattan Bridge span on the ''north'' side tracks, passes through DeKalb Avenue and then becomes the BMT Brighton Line for that line's entire distance to Stillwell Avenue. BMT A originally traversed the entire north side of the bridge, connecting Canal to DeKalb, while H ran towards Chambers St. instead of Canal, but this was reconfigured as part of the Chrystie Street Connection. The IND B designation was extended past the portal of the new connection, to meet the BMT A in the middle, yet retained the BMT chaining numbers on the bridge itself. The tracks leading to Chambers St. were severed from the bridge and were re-designated as part of BMT J.

In a few cases, the chaining lines and service letters are coincident, such as the IND A chaining line following the A service from 207th Street to Euclid Avenue, while the IND C was the IND Concourse Line, which was originally served by the C service. However, that service was later moved off of the Concourse line, while the chaining code remains.

Chaining stations

Each specific location along a line is known as a ''chaining station'', and is identified by a number unique to that ''chaining line''. The precision of the location depends on its usage. On engineering maps, the location of such features as curves, switches, crossings, stations and platforms are ordinarily specified to a precision of . This is expressed as 'chains ''plus'' feet'' a chaining station located 1,470 feet from chaining zero would be described as 14+70. For greater precision, or where style or protocol requires it, unit of less than a foot may be described. The Brooklyn Rapid Transit Company used 'chains_''plus''_feet_''point''_ 'chains_''plus''_feet_''point''_hundredths''">hundredth.html"_;"title="'chains_''plus''_feet_''point''_hundredth">'chains_''plus''_feet_''point''_hundredths''without_trailing_zeros:_thus_a_map_location_designated_as_P.S._14+70.25_would_indicate_that_the_Point_of_a_Switch_at_that_location_is_1,470_and_one_quarter_feet_(1,470_feet_and_3_inches)_from_chaining_zero.

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