The BMT Nassau Street Line is a rapid transit line of the B Division of the New York City Subway system in Manhattan. At its northern end, the line is a westward continuation of the BMT Jamaica Line in Brooklyn after the Jamaica Line crosses the Williamsburg Bridge into Manhattan. The Nassau Street Line continues south to a junction with the BMT Broadway Line just before the Montague Street Tunnel, after which the line reenters Brooklyn. Although the tracks merge into the Broadway Line south of Broad Street, there has been no regular service south of the Broad Street station since June 25, 2010. While the line is officially recognized as the Nassau Street Line, it only serves one station on Nassau Street: Fulton Street.

The BMT Jamaica Line, also known as the Broadway - Brooklyn Line, is an elevated rapid transit line of the B Division of the New York City Subway, in Brooklyn and Queens, New York City, United States. It runs from the Williamsburg Bridge southeast over Broadway to East New York, Brooklyn, and then east over Fulton Street and Jamaica Avenue to Jamaica, Queens. In western Jamaica, the line goes into a tunnel, becoming the lower level of the Archer Avenue lines in central Jamaica. The J and Z trains serve the entire length of the Jamaica Line, and the M serves the line west of Myrtle Avenue.