Complex sentence: The principal entrance, at the south end of the station, is from 86th Street and Fourth Avenue; stairs lead down to a fare control area and mezzanine, which has historically had a modest retail space (unused as of December 2005).
Split into simpler sentences: The principal entrance, at the south end of the station, is from 86th Street and Fourth Avenue. Stairs lead down to a fare control area and mezzanine, which has historically had a modest retail space (unused as of December 2005).