It is one of several lines that serves the A Division, stretching from South Ferry in Lower Manhattan north to Van Cortlandt Park–242nd Street in Riverdale, Bronx.

The line was constructed in two main portions by the Interborough Rapid Transit Company (IRT), a private operator.

To allow the wide four-track line to go through the area, new streets had to be mapped and built, and new buildings were constructed as a result.

Originally, express and local trains ran to both the Broadway Branch and to the Lenox Avenue Line, resulting in delays.

Also known as the IRT West Side Line,[6] since it runs along the west side of Manhattan, the IRT Broadway–Seventh Avenue Line runs from Van Cortlandt Park–242nd Street in the Bronx, close to New York City's border with Westchester, to South Ferry in Lower Manhattan, at the southernmost point in the borough.

Along the way, the line serves places such as Times Square, Lincoln Center, Columbia University, and the City College of New York.

[8]: 162–191 Train services that use the IRT Broadway–Seventh Avenue Line are colored red on subway signage and literature.

[13] The Clark Street Tunnel carries the 2 and ​3 trains under the East River between the boroughs of Manhattan and Brooklyn.

It was opened for revenue service on Tuesday, April 15, 1919, relieving crowding on the Joralemon Street Tunnel and providing passengers with a direct route between Brooklyn and the west side of Manhattan.

[17] Booth & Flinn Ltd. and the O'Rourke Engineering Construction Company received a $6.47 million contract in July 1914 to build a tunnel between Old Slip in Manhattan and Clark Street in Brooklyn.

[24][25] On December 28, 1990, an electrical fire in the Clark Street Tunnel trapped passengers on a subway train for over half an hour.

[26] Operation of the first subway began on October 27, 1904, with the opening of all stations from City Hall to 145th Street on the West Side Branch.

[38][39] (The original plan had been to turn east on 230th Street to just west of Bailey Avenue, at the New York Central Railroad's Kings Bridge station.

[17] It was predicted that the subway extension would lead to the growth of the Lower West Side, and to neighborhoods such as Chelsea and Greenwich Village.

After going through Lower Manhattan, the second branch would go through a tunnel under the East River before running under Clark and Fulton Streets until a junction at Borough Hall with the existing Contract 2 IRT Brooklyn Line.

[47][48] In order to pass under the Broadway and Park Row subway lines, this branch has grades as steep as 3%, being located 60 feet (18 m) below surface level.

Seven people were killed after a blast of dynamite in the subway tunnel destroyed the plank roadway over Seventh Avenue.

As a result, a crowded trolley car, and a brewery truck fell into the excavation, accounting for most of the injuries.

[51][52] This short extension was opened even though the rest of the route was not yet completed in order to handle the mass of traffic to and from Pennsylvania Station.

Only the northern part of the station was opened at this time, and piles of plaster, rails, and debris could be seen on the rest of the platforms.

[55] The new "H" system was implemented on August 1, 1918, joining the two halves of the Broadway–Seventh Avenue Line and sending all West Side trains south from Times Square.

[6] In 1955, the firm Edwards, Kelcey and Beck was hired as Consulting Engineers for the construction of the express station.

[63] Under a $100 million (equivalent to $1,045,205,479 in 2023) rebuilding program, increased and lengthened service was implemented during peak hours on the 1 train.

[71] In 1986, the NYCTA launched a study to determine whether to close 79 stations on 11 routes, including the Broadway–Seventh Avenue Line north of 215th Street, due to low ridership and high repair costs.

[72][73] Numerous figures, including New York City Council member Carol Greitzer, criticized the plans.

[78] After the September 11 attacks, all 1 trains had to be rerouted since the IRT Broadway–Seventh Avenue Line ran directly under the World Trade Center site and was heavily damaged in the collapse of the Twin Towers.

As a condition of the funding allocation for the station renovation at 103rd Street, the university wanted work on the project to be expedited.

Residents of Morningside Heights approved of the renovations plans, but were concerned that the expedited repairs would come at the cost of damaging the stations' historic elements.

The MTA was expected to decide whether preservation or speed would be prioritized in the station renovation projects by the end of the year.

The MTA had planned to install a small bronze subway track and train to be inlaid within the station walls surrounded by sepia-toned photographs of the neighborhood at 116th Street.