on October 29, 1989, and was connected to the Queens Boulevard Line on December 16, 2001. The BMT 63rd Street Line in Manhattan was only used for regular service starting in 2017, when the Second Avenue Subway's first phase opened. Extent and service The following services use the 63rd Street Lines: The 63rd Street Lines comprise two physical pairs of trackage; each track pair's mileage is measured via different subway chaining schemes. One is chained as part of the Independent Subway System (IND), and the other is chained as part of the Brooklyn–Manhattan Transit Corporation (BMT). The IND line begins as a northward continuation of the IND Sixth Avenue Line at 57th Street station. It runs under Sixth Avenue and Central Park, turning east under 63rd Street and running through the 63rd Street Tunnel under the East River, with stations at Main Street on Roosevelt Island and at 21st Street under 41st Avenue in Queens. At its eastern end, the line merges with the IND Queens Boulevard Line under Northern Boulevard, west of 36th Street station. This line is coded as chaining route "T", with the southbound track marked as T1 and the northbound track designated T2. Beneath the subway tunnel is an unused lower level that is reserved for the Long Island Rail Road (LIRR). It is being connected to the Grand Central Terminal as part of the LIRR's East Side Access project, which is expected to commence operations by 2022. The BMT line begins as a northward continuation of the express tracks of the BMT Broadway Line at the 57th Street–Seventh Avenue station. It runs under Seventh Avenue, Central Park and 63rd Street before turning north under Second Avenue and merging with the Second Avenue Subway. The tracks on this line are coded with BMT chaining, labeled as tracks G3 and G4 to distinguish them from the pre-existing G1 and G2 tracks associated with the 60th Street Tunnel and Astoria Line. The odd-numbered tracks carry southbound trains, while the even-numbered tracks carry northbound trains. The two lines intersect at the Lexington Avenue–63rd Street station, where there are cross-platform interchanges for both northbound and southbound trains. Just west of Lexington Avenue-63rd Street, two diamond crossover tracks allow trains to switch between the two lines. This connection, not used in passenger service, allows trains to run from the Broadway Line to the Queens Boulevard Line and from the Sixth Avenue