The IND 63rd Street Line and BMT 63rd Street Line, also referred to as the 63rd Street Crosstown, Crosstown Route, or Route 131-A, are two rapid transit lines of the B Division of the New York City Subway system. The two lines run under 63rd Street in the Upper East Side of Manhattan, with a cross-platform interchange at the Lexington Avenue–63rd Street station.
Each line consists of two tracks in a stacked configuration, with the southbound tracks of each line parallel to each other on the upper level, while the northbound tracks are parallel on the lower level. Crossover connections between each line's respective tracks are located just to the west of the Lexington Avenue station. The IND line, the southern of the two lines, is served by F trains at all times; it connects the IND Sixth Avenue Line in Manhattan to the IND Queens Boulevard Line in Queens. It uses the upper level of the bi-level 63rd Street Tunnel to travel under the East River between Manhattan and Queens. The northern, BMT, line is served by the Q train at all times, although a limited number of N and R trains also serve the line during rush hours. Also known as the Second Avenue Connection,: 128 it links the BMT Broadway Line to the Second Avenue Subway, both in Manhattan. The crossover between the lines been used in the past during service disruptions as well as during the Manhattan Bridge closures in the late 1990s.
These lines were conceived as part of the Metropolitan Transportation Authority's 1968 expansion plans, and along with the Archer Avenue lines and a small section of the Second Avenue Subway, they were the only portions of the plan to be completed before it was scaled back due to fiscal issues.