6 was the BMT

Brooklyn-Manhattan Transit Corporation

The Brooklyn–Manhattan Transit Corporation was an urban transit holding company, based in Brooklyn, New York City, United States, and incorporated in 1923. The system was sold to the city in 1940 and today, together with the IND subway system, form the B Division of the New York City Subway...

's designation for trains that used the BMT Fifth Avenue Line (labeled the Fifth Avenue–Bay Ridge Line).

When numbers were assigned in 1924, 6 was assigned to trains between Sands Street (on the Brooklyn side of the Brooklyn Bridge

Brooklyn Bridge

The Brooklyn Bridge is one of the oldest suspension bridges in the United States. Completed in 1883, it connects the New York City boroughs of Manhattan and Brooklyn by spanning the East River...

) and 65th Street

65th Street Terminal (BMT Fifth Avenue Line)

65th Street Terminal was a station on the demolished section of the BMT Fifth Avenue Line. It was served by trains of the BMT Fifth Avenue Line. It had two tracks and one island platform. The next stop to the north was 59th Street. It closed on May 31, 1940. Today, the Gowanus Expressway and Belt...

, via the BMT Myrtle Avenue Line

BMT Myrtle Avenue Line

The Myrtle Avenue Line, also called the Myrtle Avenue Elevated, is a fully elevated line of the New York City Subway as part of the BMT division. The extant line is the final remnant of one of the original Brooklyn elevated railroads...

, BMT Third Avenue Line, and BMT Fifth Avenue Line.

Between 1931 and 1937, all service except rush hour and Saturday morning service was truncated to 36th Street

36th Street (BMT Fifth Avenue Line)

36th Street was a station on the demolished section of the BMT Fifth Avenue Line. It was served by trains of the BMT Culver Line and BMT Fifth Avenue Line. It had four tracks and two island platforms. The next stop to the north was 25th Street. The next stop to the south was Ninth Avenue for Culver...

, where transfer could be made to a Culver Line

IND Culver Line

The IND Culver Line is a rapid transit line of the B Division of the New York City Subway, extending from Downtown Brooklyn south to Coney Island, Brooklyn, New York City, United States...

 (5) train to downtown Brooklyn. On May 31, 1940, the Third Avenue and Fifth Avenue lines were closed, ending all 6 service.