BMT Fourth Avenue Line

The BMT Fourth Avenue Line is a rapid transit line of the New York City Subway, mainly running under Fourth Avenue in Brooklyn. The line is served by the D, N, and R at all times; the R typically runs local, while the D and N run express during the day and local at night. During rush hours, select W trains also serve the line. Northbound D and N trains run local along the line after 6:45 p.m. during weekdays as well. The line was originally built by the Brooklyn Rapid Transit Company (BRT; later Brooklyn–Manhattan Transit Corporation, or BMT) and is now internally operated as part of the New York City Subway's B Division.

The fully underground line starts as a two-track line in Downtown Brooklyn west of Court Street, connecting to the BMT Broadway Line and BMT Nassau Street Line in Manhattan via the Montague Street Tunnel under the East River. It travels east under Montague and Willoughby Streets to DeKalb Avenue, where it then turns southeast under Flatbush Avenue. At DeKalb Avenue, the express tracks, which continue from the Manhattan Bridge to the northwest, split off from the BMT Brighton Line and join the Fourth Avenue Line. At Atlantic Avenue–Barclays Center, the line curves southwest under Fourth Avenue to the end of the line at Bay Ridge–95th Street. Going south from Atlantic Avenue, the BMT West End Line splits from both the local and express tracks south of 36th Street, while the express tracks continue as the BMT Sea Beach Line south of 59th Street.

Fourth Avenue never had a streetcar line or elevated railway due to the provisions of the assessment charged to neighboring property owners when the street was widened. Construction of the line was only undertaken because of the efforts of the local communities. After the line was opened, development resulting from the line's construction transformed communities such as Dyker Heights, Fort Hamilton, Bay Ridge, and Sunset Park. One station, Myrtle Avenue, was abandoned in 1956 as part of the reconfiguration of the busy DeKalb Avenue Junction.

Coming south from DeKalb Avenue and off of Fulton Street, the four-track line runs entirely under Fourth Avenue to just past 59th Street. South of 36th Street, the West End Line, formerly the New Utrecht Avenue elevated line, branches off eastward, running to its terminus at Coney Island–Stillwell Avenue. Until 1954, the BMT Culver Line also branched off from here, replaced by the Culver Shuttle until 1975, when it was discontinued. At 64th Street, after the Sea Beach Line branches off eastward towards Coney Island via an open-cut right-of-way, the line becomes two-tracked and continues under Fourth Avenue to its terminus at Bay Ridge–95th Street. Between 86th Street and 95th Street there is a third track available for train storage. While this section of the line was built with two tracks, there are provisions to add two additional express tracks between 59th and 85th Streets if the need ever arises.

The line's northernmost section serves Brooklyn Heights and Downtown Brooklyn. The central section serves Park Slope east of Fourth Avenue and north of Prospect Expressway; Boerum Hill and Gowanus west of Fourth Avenue and north of Prospect Expressway; and Sunset Park to the south of Prospect Expressway. The southernmost section serves the communities of Bay Ridge and Fort Hamilton.

The following services use the Fourth Avenue Line:

The line normally carries the Fourth Avenue R service on the local tracks and the Sea Beach N and West End D services on the express tracks. During weekdays, five local N trains per day are designated as W trains, Manhattan-bound from DeKalb Avenue, the local tracks run via the Montague Street Tunnel to Lower Manhattan, serving either Whitehall Street on the BMT Broadway Line (N, R) or Broad Street on the BMT Nassau Street Line (no service; the last service to use these tracks was the M, rerouted in 2010). The express tracks go to Manhattan via the Manhattan Bridge to either the BMT Broadway Line's Canal Street express station (N) or the IND Sixth Avenue Line's Grand Street station (D).

Transportation to the area was first provided in 1889 with the establishment of the 39th Street Ferry, which connected the area to Manhattan. Between 1888 and 1893, a new elevated line was opened along Fifth Avenue. Initially, the line terminated at 27th Street where people could transfer to horse cars. In 1892, the first trolley line was built in Brooklyn, starting at the ferry and running via Second Avenue to 65th Street, and then via Third Avenue. The Fifth Avenue Elevated was then extended to Third Avenue and 65th Street.