Since the opening of the original New York City Subway line in 1904, and throughout the subway's history, various official and planning agencies have proposed numerous extensions to the subway system. The first major expansion of the subway system was the Dual Contracts, a set of agreements between the City of New York and the IRT and the BRT. The system was expanded into the outer reaches of the Bronx, Brooklyn, and Queens, and it provided for the construction of important lines in Manhattan. This one expansion of the system provided for a majority of today's system.
Even with this expansion, public officials wanted more. In 1922, Mayor John Hylan put out his plan for over 100 miles of new subway lines going to all five boroughs. His plan was intended to directly compete with the two private subway operators, the IRT and the BMT. This plan was never furthered. The next big plan, and arguably the most ambitious in the subway system's history, was the "Second System." The 1929 plan by the Independent Subway to construct new subway lines, the Second System would take over existing subway lines and railroad right-of-ways. This plan would have expanded service throughout the city with 100 miles of subway lines. A major component of the plan was the construction of the Second Avenue Subway. The Stock Market Crash of 1929 put a halt to the plan, however, and subway expansion was limited to lines already under construction by the IND.
During the 1930s and 1940s, the plans were revised, with new plans such as a line to Staten Island and a revised line to the Rockaways. In the late 1940s and 1950s, a Queens Bypass line via the Long Island Rail Road's Main Line was first proposed as a branch of the still-planned Second Avenue Subway. In addition, capacity on existing lines would have been improved through the construction of strategic connections such as the Culver Ramp, the 60th Street Tunnel Connection, and the Chrystie Street Connection, and through the rebuilding of DeKalb Avenue Junction. These improvements were the only things to come out of these plans. Eventually, these plans were modified to what became the Program for Action, which was put forth by the New York City Transit Authority in 1968. This was the last plan for a major expansion of the subway system. The plan included the construction of the Second Avenue Subway, a Queens Bypass line, a line replacing the Third Avenue El in the Bronx, and other extensions in the Bronx, Queens, and Brooklyn. While ambitious, very little of the plan was completed, mostly because of the financial crisis during the 1970s.
Until the 1990s, there was little focus on expansion of the system because the system was in a state of disrepair, and funds were allocated to rebuilding and repairing the existing system. In the 1990s, however, with the system in better shape, the construction of the Second Avenue Subway was looked into again. Construction of the Second Avenue Subway started in 2007, and the first phase was completed in 2017. Since the 1990s, public officials and organizations such as the Regional Plan Association have pushed for the further expansion of the system. Projects such as the TriboroRx, a circumferential line connecting the outer boroughs, the reuse of the Rockaway Beach Branch, and the further expansion of the Second Avenue Subway have all been proposed, albeit unfunded.
The Triborough System was a proclamation for new subway lines to the Bronx and Brooklyn. The new lines include the IRT Lexington Avenue Line, IRT Pelham Line, and IRT Jerome Avenue Line. The Manhattan Bridge line described below later became the BMT West End Line, BMT Fourth Avenue Line, the BMT Sea Beach Line, and the Nassau Street loops.
The route of the new subway ... comprises a main trunk north and south through Manhattan Borough on Lexington Avenue and Irving Place from the Harlem River to Tenth St. and on Broadway, Vesey and Church Sts. from Tenth St. to the Battery; two branches in Bronx Borough, one northeast via 138th St. Southern Boulevard and Westchester Ave. to Pelham Bay Park. the other northerly via River Ave. and Jerome Ave.. to Woodlawn Road, connecting with the Manhattan trunk by a tunnel under the Harlem River; a Manhattan-Brooklyn line extending from the North River via Canal Street across the East River on the Manhattan Bridge to connect with the Fourth Avenue subway in Brooklyn now being built, which thus becomes an integral part of the larger system; two branches southerly from the Fourth Ave. line extending south to Fort Hamilton and southeast to Coney Island; and a loop feeder line in Brooklyn through Lafayette Ave. and Broadway, connecting with the Fourth Ave. line at one end. and at the other crossing the Williamsburg Bridge and entering the Centre Street Loop subway in Manhattan which is thus also incorporated in the system.
In 1911, William Gibbs McAdoo, who operated a competing subway company called the Hudson & Manhattan Railroad, proposed building a line under Broadway between Hudson Terminal and Herald Square. He later proposed that the Broadway line be tied into the IRT's original subway line in Lower Manhattan. The Broadway line, going southbound, would merge with the local tracks of the IRT Lexington Avenue Line in the southbound direction at 10th Street. A spur off the Lexington Avenue Line in Lower Manhattan, in the back of Trinity Church, would split eastward under Wall Street, cross the East River to Brooklyn, then head down the Fourth Avenue Line in Brooklyn, with another spur underneath Lafayette Avenue.
The Triborough System later became part of the Dual Contracts, signed on March 19, 1913 and also known as the Dual Subway System. These were contracts for the construction and/or rehabilitation and operation of rapid transit lines in New York City. The contracts were "dual", in that they were signed between the City and the IRT and Municipal Railway Company, a subsidiary of the BRT (later BMT).