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.
Chaining numbers can be seen on the black pillars in the middle of the tracks of West Fourth Street–Washington Square.
The IRT Broadway–Seventh Avenue Line is a New York City Subway line. It is one of several lines that serves the A Division, stretching from South Ferry in Lower Manhattan north to Van Cortlandt Park–242nd Street in Riverdale, Bronx. The Brooklyn Branch, known as the Wall and William Streets Branch during construction, from the main line at Chambers Street southeast through the Clark Street Tunnel to Borough Hall in Downtown Brooklyn, is also part of the Broadway–Seventh Avenue Line. The IRT Broadway–Seventh Avenue Line is the only line to have elevated stations in Manhattan, with two short stretches of elevated track at 125th Street and between Dyckman and 225th Streets.
50th Street, one of the line's original stations
Emergency exit, Furman Street, Brooklyn
1915 Seventh Avenue subway collapse with car fallen in tunnel
A 1 train in service departing 125th Street along part of the route of the Original Subway.