IRT Lexington Avenue Line

The IRT Lexington Avenue Line (also known as the IRT East Side Line and the IRT Lexington–Fourth Avenue Line) is one of the lines of the A Division of the New York City Subway, stretching from Lower Manhattan north to 125th Street in East Harlem. The line is served by the 4, ​5, ​6, and <6> trains.

The line was constructed in two main portions by the Interborough Rapid Transit Company (IRT), a private operator. The first portion, from City Hall north to 42nd Street, was opened between 1904 and 1908, and is part of the first subway line in the city. The original subway turned west across 42nd Street at the Grand Central station, then went north at Broadway, serving the present-day IRT Broadway–Seventh Avenue Line. The second portion of the line, north of 42nd Street, was constructed as part of the Dual Contracts, which were signed between the IRT; the Brooklyn Rapid Transit Company, via a subsidiary; and the City of New York.

For decades, the Lexington Avenue Line was the only line in Manhattan that directly served the Upper East Side and East Midtown; this four-track line is the most used rapid transit line in the United States. Its average of 1.3 million daily riders is more than the total riderships of the transit systems of Chicago (772,900 weekday passengers), Boston (569,200 weekday passengers), and San Francisco (452,600 weekday passengers). The line spurred the construction of the parallel Second Avenue Subway, which opened in 2017, to relieve congestion on the Lexington Avenue line.

Four stations along this line have been abandoned. When platforms were lengthened to fit ten cars, it was deemed most beneficial to close these stations and open new entrances for adjacent stations. The 18th Street station was abandoned because of the proximity to both 14th Street–Union Square and 23rd Street. In addition, the City Hall and Worth Street stations were both very close to the Brooklyn Bridge–City Hall station's Brooklyn Bridge and Duane Street exits, respectively, so both were abandoned. Finally, South Ferry is within walking distance of Bowling Green, and is right next to the corresponding station on the Broadway–Seventh Avenue line.