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 and public maps and timetables. These latter 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.
Read more about this topic: New York City Subway Chaining
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