The line was originally proposed in 1920 as part of a massive expansion of what would become the Independent Subway System (IND). In anticipation of the Second Avenue Subway being built to replace them, parallel elevated lines along Second Avenue and Third Avenue were demolished in 1942 and 1955, respectively, despite several factors causing plans for the Second Avenue Subway to be cancelled. Construction on the line finally began in 1972 as part of the Program for Action, but was halted in 1975 because of the city's fiscal crisis, leaving only a few short segments of tunnels completed. Work on the line restarted in April 2007 following the development of a financially secure construction plan. The first phase of the line, consisting of the 96th Street, 86th Street and 72nd Street stations, as well as of tunnel, cost $4.45 billion. A , $6 billion second phase from 96th to 125th Streets is in planning and is expected to open by 2027–2029.