12 was the Brooklyn-Manhattan Transit Corporation's designation for trains that used the BMT Lexington Avenue Line.
In addition, some afternoon rush hour trains were added between Sands Street (on the Brooklyn side of the Brooklyn Bridge) and 111th Street.
All Lexington Avenue trains were once again extended across the Brooklyn Bridge to Park Row, and a new service called Fulton – Lexington Avenue was inaugurated, replacing service that had been provided by Fulton Street trains, and operating to the Fulton Street Line terminus at Lefferts Avenue.
On March 5, 1944, the BMT Myrtle Avenue Line was closed west of Jay Street and the Bridge Street station at that location was renamed Bridge–Jay Streets; all 12 trains were truncated there (with a free transfer first to streetcars, and then to the IND at Jay Street – Borough Hall) – see List of New York City Subway transfer stations for details.
The BMT Lexington Avenue Line closed on October 13, 1950, ending all 12 service.