Unification and Contraction
In June 1940, the transportation assets of the former BMT and IRT systems were taken over by the City of New York for operation by the City's Board of Transportation, which already operated the IND system. In 1953 the New York City Transit Authority, a state agency incorporated for the benefit of the city, now known to the public as MTA New York City Transit, succeeded the BoT.
A combination of factors had this takeover coincide with the end of the major rapid transit building eras in New York City. The City immediately began to eliminate what it considered redundancy in the system, closing several elevated lines including the IRT Ninth Avenue Line and most of the IRT Second Avenue El in Manhattan, and the BMT Fifth and Third Avenue Lines and most of the BMT Fulton Street Line in Brooklyn.
Despite the unification, a distinction between the three systems survives in the service labels: IRT lines (now referred to as A Division) have numbers and BMT/IND (now collectively B Division) lines use letters. There is also a more physical but less obvious difference: Division A cars are narrower than those of Division B by 18 inches (~45 cm) and shorter by 9 to 24 feet (~2.7 to 7.3m).
The original IRT subway lines were built to modified elevated line dimensions. Whereas the IRT els were originally equipped with cars that were 47 feet (~14.3m) long, the cars designed for the IRT subway measure 51 feet 4 inches (~15.6m) long. Both sets of lines permitted cars not wider than 9 feet (~2.7m). The clearances and curves on these lines are too narrow and too sharp for any IND or BMT equipment. The later extensions of the IRT, constituting the bulk of the system, were built to BMT dimensions, and so are of a profile that could support the use of IND/BMT sized equipment. In other words, Division B equipment could operate on much of Division A if station platforms were trimmed and trackside furniture moved, thus letting Division A lines carry more passengers. However, there is virtually no chance of this happening because the older, narrower portions of Division A are centrally situated, such that it would be impossible to put together coherent through services. The most that can be reasonably hoped for is that some branch lines of Division A might be resized and attached to Division B lines. This was done with the BMT Astoria Line in Queens (which had formerly been dual-operated with normal IRT trains and special narrow BMT shuttles), and has been proposed for a connection of the Second Avenue Subway to the IRT Pelham Line in the East Bronx.
Because the Division A lines are of lower capacity for a given capital investment, all new extensions and lines built since World War II have been for Division B. Division A cars can travel on Division B lines when necessary, but are not used for passenger service on those lines due to the dangerously wide gap between the car and the station platform.
Even during World War II, which gave a reprieve to the closure of most rail transit in the US, some closures continued, including the remainder of the IRT Second Avenue Line in Manhattan (1942) and the surviving BMT elevated services over the Brooklyn Bridge (1944).
The originally planned IND system was built to the completion of its original plans after World War II ended, but the system then entered an era of deferred maintenance in which infrastructure was allowed to deteriorate, and closures of elevated lines continued. These closures included the entire IRT Third Avenue Line in Manhattan (1955) and the Bronx (1973), as well as the BMT Lexington Avenue Line (1950), much of the remainder of the BMT Fulton Street Line (1956), the downtown Brooklyn part of the BMT Myrtle Avenue Line (1969) and the BMT Culver Shuttle (1975), all in Brooklyn.
Only two new lines were opened in this era, the IRT Dyre Avenue Line (1941) and the IND Rockaway Line (1956). Both of these lines were rehabilitations of existing railroad rights-of-way rather than new construction. The former line was the City portion of the New York, Westchester and Boston Railway (an electrified commuter line closed in 1937) and the latter a line obtained from the Long Island Rail Road. While the Rockaway Line is a long and substantial line, it consists mostly of a long right-of-way crossing Jamaica Bay with a single station on Broad Channel island and two branches on a peninsula that is only several city blocks wide.
In 1951 a half-billion dollar bond issue was passed to build the Second Avenue Subway, but money from this issue was used for other priorities and the building of short connector lines, namely a ramp extending the IND Culver Line over the ex-BMT Culver Line at Ditmas and McDonald Avenues in Brooklyn (1954), allowing IND subway service to operate to Coney Island for the first time, the 60th Street Tunnel Connection (1955), linking the BMT Broadway Line to the IND Queens Boulevard Line, and the Chrystie Street Connection (1967), linking the BMT line via the Manhattan Bridge to the IND Sixth Avenue Line.
In the mid-1960s, $600,000,000 was made available to the Metropolitan Transportation Authority (MTA) of New York City for the purposes of Subway expansion. $1,230,000,000 was spent to create three tunnels and a half-dozen holes as part of construction on the Second Avenue and 63rd Street Lines. Construction would cease in 1975 on account of the city's severe fiscal crisis; none of the sections were usable by the time federal payments were suspended in 1985.
Because the early subway systems competed with each other, they tended to cover the same areas of the city, leading to much overlapping service. The amount of service has actually decreased since the 1940s as many elevated railways were torn down, and finding funding for underground replacements has proven difficult.
Due to deferred maintenance, the condition of the subway system reached dangerous conditions as of the early 1980s. Talk of new construction was considered absurd at that point. However, during the mid-1980s, reconstruction began. Stations were refurbished and rolling stock was repaired and replaced.
Read more about this topic: History Of The New York City Subway