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In the U.S., Chaining is a method by which railroads precisely measure and specify locations along the line. 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).
This article describes chaining on the New York City Subway system, where each "chain" is 100 feet (~30.5 m) long. This is as opposed to the milepost or mileage system, where distances are usually measured in miles and hundredths.
Chaining zero
Chaining zero is a fixed point from which the chaining is measured on a particular chaining line. A chaining number at a given location of 243, say, identifies that the point is 24,300 feet (~4.6 miles or ~7.4 km) 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 or route to chaining zero on a given line. However, it is not uncommon for a line's chaining numbers to refer to a chaining zero that no longer exists or along a physical line that no longer exists, because of abandonment or demolition. It is less common but not impossible for a reroute to alter the accuracy of chaining numbers, if only slightly.
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 whatever to do with the letters displayed on trains and public maps and timetables. These are subway route letters.
For example, the BMT A chaining line begins at BMT South chaining zero north of 57th Street on the Broadway-BMT 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 Brighton Line for that line's entire distance to Coney Island Terminal.
Chaining zero locations
Each division of the subway system has one or more chaining zero locations.
IND
The original IND system has only one chaining zero, and the only one based on a physical location that is not and has never been on the subway or elevated system. IND zero was calculated by extending a straight line drawn from the centerline of the West Fourth Street-Washington Square Park station in Greenwich Village to the point in Lower New York Harbor where that line intersects the New York-New Jersey border. That location, which is south of the entire City of New York, roughly north of Keansburg, New Jersey, was established as the IND chaining zero. The southern end of the Eighth Avenue Line platforms at West 4th Street station was established as chaining point 969+25, being 96,925 feet (18.357 miles) from the zero point. The entire IND system was chained by increasing the chaining numbers moving railroad north from that point, and reducing the numbers going railroad south.
When the IND Division incorporated the formerly BMT Culver Line into its system in 1954, the line was resignalled from BMT chaining line C to IND chaining line B, all the way to the end of Culver chaining at the approach to West Eighth Street-New York Aquarium and the chaining distances recalculated from IND zero.
When the IND Division was extended out the former BMT Fulton Street Elevated in Queens in 1956, the BMT chaining, both zero point and BMT chaining letter (K), were kept intact. The Rockaway Line, which was opened at the same time as part of the IND, retained the Long Island Rail Road chaining distances, measured from Long Island City LIRR station via the old Montauk and Rockaway Bay lines.
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