City water supply system. The section north of 33rd Street had mostly been excavated by November 1937. By the beginning of the following year, the track junction north of the station was being constructed at a rate of per day. The IRT's Sixth Avenue elevated closed in December 1938, just before the Sixth Avenue subway was completed. North of 47th–50th Streets, bellmouth tunnels were built to allow for a future extension under Central Park and along Morningside Avenue to 145th Street. The unbuilt extension was part of the Board of Transportation's long-range program and was budgeted at $34.914 million in August 1940. Construction was expected to start some time after 1946. The bellmouths diverged from the local and express tracks on both the northbound and southbound sides of the station, with tunnels that stub-ended at 53rd Street. In the meantime, all express trains originally turned west toward the Seventh Avenue station and the Washington Heights Line, while local trains turned east toward the Fifth Avenue/53rd Street station and the Queens Boulevard Line. Opening and use 1940s to 1960s The station opened on December 15, 1940, as part of the mainline portion of the IND Sixth Avenue Line to West Fourth Street–Washington Square. At the time, it was known as 47th Street–50th Street–Rockefeller Center. On the line's opening day, the Rockefeller Center station was the busiest of the six new stations. There were connections from the station's mezzanine to the basements of 1230 Sixth Avenue and 30 Rockefeller Plaza. The opening of the Sixth Avenue Line relieved train traffic on the Eighth Avenue Line, which was used by all IND services except for the G Brooklyn–Queens Crosstown service. The BB train served part of the Sixth Avenue Line's midtown section during weekday rush hours only, running local between 168th Street and 34th Street–Herald Square. The entirety of the midtown section was served at all times by the D and F trains. In early 1949, the BOT announced that an automatic shoe-shine machine would be installed at the station as part of a pilot program. To improve cleanliness, the BOT also installed a "hygiaphone" at the station agent's booth; the device was composed of a transparent membrane and was meant to reduce the spread of germs. Rockefeller Center Inc., the owners of Rockefeller Center, announced in 1956 that it would construct the Time–Life Building at 1271 Avenue of the Americas, with a