The entire length is underground, though the 207th Street Yard, which branches off near the north end, is on the surface.

[9] The whole line is served at all times by the A train, which runs express except during late nights.

[9] The small 174th Street Yard lies under Broadway, with two tracks exiting to the south under that roadway.

Approaching 59th Street–Columbus Circle, where Central Park West becomes Eighth Avenue, the subway again spreads out into a single four-track level.

An unused southbound-only lower level at 42nd Street–Port Authority Bus Terminal was formerly accessed only from the southbound track from the Queens Boulevard Line.

Plans for the 7 Subway Extension required partially demolishing the lower level to make room for the new IRT Flushing Line tracks.

The two-level West Fourth Street–Washington Square station allows easy transfers between the two lines.

The Sixth Avenue Line turns east into Houston Street after passing the connections.

At this point, a bellmouth originally intended for the never-built IND Worth Street Line is present on the east side of the tunnel.

[9] As early as March 1918, soon after the BMT Broadway Line opened to Times Square–42nd Street, plans were being considered for an extension of that line beyond the stubs at 57th Street–Seventh Avenue to the Upper West Side and Washington Heights via Central Park West (Eighth Avenue).

[18][19] Mayor John Hylan instead wanted to build an independent subway system, operated by the city.

The New York City Board of Transportation (NYCBOT) gave preliminary approval to several lines in Manhattan, including one on Eighth Avenue, on December 9, 1924.

[22] Most of the Eighth Avenue Line was dug using a cheap cut-and-cover method, where the street above was excavated.

At 59th Street–Columbus Circle, workers had to be careful to not disrupt the existing IRT Broadway–Seventh Avenue Line overhead.

[23] In the summer of 1926 the BOT held a public hearing and agreed upon the details of the construction of the subway line under Church Street.

[28] The city condemned entire lots, displacing 10,000 people,[29] to build the extension and used leftover land for parks.

It was proposed to develop the mezzanines of these four stations with shops, so that they would become retail corridors, similar to the underground mall of the under-construction Rockefeller Center.

Both express and local trains would run at intervals of four to twelve minutes depending on the time of day.

[38] The E was added to the local tracks south of 50th Street on August 19, 1933, when the IND Queens Boulevard Line opened.

[39] The final major change came on December 15, 1940, when the IND Sixth Avenue Line opened.

The CC was kept only during rush hours to provide local service south past 59th Street.

Additionally the C became a rush hour-only service, replaced by a full-time D over the express tracks between the Concourse and Sixth Avenue Lines.

The A began making local stops in Manhattan during late nights, when the AA was not running.

[44] On December 10, 1988, the K designation was discontinued and merged into the C, which now ran at all times except late nights.

[52] The B and the C, which both ran local along Central Park West, switched northern terminals on March 1, 1998, ending the connection between the C and the Bronx.

[68]: 8  Construction was originally scheduled to begin in October 2018,[67] but a contract for CBTC installation was later pushed back to the first quarter of 2019.

[80] At the March 12 hearing, the project's construction was endorsed by east side civic organizations.

[81] In June 1930, the Board of Estimate approved the construction of the line, and in July Chairman Delaney sent letters to 450 real estate owners outlining the planned route and requested their consent for the construction of the project, of which the Board needed 50%.

The Board of Transportation resumed efforts to build the line after a study of existing conditions in the area was completed, and on the basis that funding would be provided from government and private sources.

The Lewis Street stop, located in the Lower East Side, would have been the line's terminal.