14th Street–Union Square is an express station on the IRT Lexington Avenue Line. The 4 and 6 trains stop here at all times; the 5 train stops here at all times except late nights; and the 6d (New York City Subway service) train stops here during weekdays in the peak direction. The station has four tracks and two island platforms. The uptown and downtown platforms are offset from each other, having been extended at their rear ends, and are slightly curved. Platform gap fillers, on the downtown side, use proximity sensors to detect when trains arrive, automatically extending when a train has stopped in the station.