B
The B line opened on December 15, 1940, with rush hour service between 168th Street, Manhattan to 34th Street - Herald Square. On November 26, 1967, the Chrystie Street Connection opened, which allowed for a train to travel from 168th Street in Manhattan to Coney Island in Brooklyn. Over the years, due to budget cuts from the city, the B went back and forth between fully running and acting as a shuttle between 36th Street, Brooklyn, to Coney Island.
In 1989, the IND 63rd Street Line opened (serviced by the F and the Q), which extended the service of the B line from 57th Street to 21st Street–Queensbridge. On March 1, 1988, the B and C lines switched northern terminals, making the C terminate service at 168th Street, and the B terminate service at Bedford Park Boulevard.
D
The IND Sixth Avenue line opened on December 15, 1940, creating the B, D, and F lines. The original route of the D line was from 205th Street in the Bronx to World Trade Center in Manhattan. Below West Fourth Street, the D would interchange between running on the Sixth Avenue Line and the Eighth Avenue Line (usually serviced by the A, C, or E trains). In 1954, the Culver Ramp opened, connecting the IND South Brooklyn Line and BMT Culver Line, extending the D train service to Coney Island in Brooklyn. With the 1967 opening of the Chrystie Street Connection, the D train took over the route the Q train had in Brooklyn. In Manhattan, the D and B run on the same line, making all stops, but the D train goes express after 59th Street - Columbus Circle.
F
Like the B and D trains, the F line started with the opening of the IND Sixth Avenue Line, but the F serviced Queens Boulevard. It ran between Parsons Boulevard in Queens to Church Avenue in Brooklyn. The line ran express in Queens but local in Manhattan and Brooklyn. The line’s service was extended on January 10, 1944, to 169th Street, Queens, during evenings, late nights, and Sunday mornings. It had its final extension on December 11, 1950, to Jamaica - 179th Street, where it terminates today. Just like the E train, in 1953, the F’s tracks were lengthened to have 11 cars per train, increasing passenger capacity by 4,000.
M
The M is the only train whose tracks run through two boroughs with unconnected lines. Its southern terminal and northern terminal are both in Queens but do not connect. Originally called the 10, the M began on July 29, 1914, as a track connecting Myrtle Avenue to the BMT Broadway Elevated. In 1961, letter designations for trains were adopted, and the 10 became the M. In 1968, the second half of the Chrystie Street Connection opened, and the M was extended to Chambers Street and Broad Street. It was adapted onto the Brighton line, stretching it from Broad Street down to Coney Island. With its southern terminal at Forest Hills - 71st Avenue, in 2017, the northern terminal was moved to Metropolitan Avenue rather than it previously diverging at Myrtle Avenue.
