When traveling along the downtown Lexington Avenue (4/5/6) platform at 14th Street-Union Square, one may hear a loud bang every time a train enters or exits the station. These are the moving gap fillers, meant to bridge a gap between a train and platform that would otherwise snag unsuspecting commuters.

The station is part of the original subway line from 1904 and the track is slightly curved. This wasn’t a problem with the original car designs, which had their doors at the ends of every car. The doors lined up neatly with the platform and everything was fine until the subway switched to trains with middle doors, as well as back doors that were further from the end. This necessitated creating gap fillers for any riders who chose to board or disembark via these doors.

Union Square wasn’t the only station equipped with these gap fillers at first, with stations like South Ferry and the Times Square shuttle also employing them. But those stations were renovated in 2009 and 2021 respectively, leaving Union Square’s downtown 4/5/6 platform as the sole survivor. 

The hydraulic gap fillers operate via a motion sensor, automatically extending when trains enter the station and retracting when the train leaves. The fillers are clearly marked on the floor so that passengers know to stand clear.