The express tracks from Columbus Circle then turn south to go under Sixth Avenue, merging with the branch from 57th Street and the local tracks' split from the IND Queens Boulevard Line (used by the M local train). The branch from 57th Street merges into both the local and express track pairs; there are no direct track connections between the local and express tracks. South of this point, the Sixth Avenue Line consists of four tracks from west to east: the southbound express track, the southbound local track, the northbound express track, and the northbound local track. After passing through 47th–50th Streets–Rockefeller Center, the two southbound tracks cross each other; the tracks from Columbus Circle become the two center express tracks, and the tracks from the Queens Boulevard Line are the two outside local tracks.