In 1940, the subway system was unified, with the IRT and the BMT coming under city control. Consequently, plans for the proposed line were modified. The current plans were drawn up in the 1960s under the Metropolitan Transportation Authority's Program For Action. Under this plan, the line was to connect to the IND Sixth Avenue and BMT Broadway Lines. The IND line was to be built on the upper portion of the bi-level 63rd Street Tunnel, which would run under the East River. As such, the tracks connecting to the IND Sixth Avenue Line comprise the IND 63rd Street Line, on the south side of the station, while the tracks connecting to the BMT Broadway Line comprise the BMT 63rd Street Line, on the north side of the station.