IRT Lexington Avenue Line in the context of "6d (New York City Subway service)"

The first regularly operated line of the New York City Subway was opened on October 27, 1904, and was operated by the Interborough Rapid Transit Company (IRT). The early IRT system consisted of a single trunk line running south from 96th Street in Manhattan (running under Broadway, 42nd Street, Park Avenue, and Lafayette Street), with a southern branch to Brooklyn. North of 96th Street, the line had three northern branches in Upper Manhattan and the Bronx. The system had four tracks between Brooklyn Bridge–City Hall and 96th Street, allowing for local and express service. The original line and early extensions consisted of:

• The IRT Eastern Parkway Line from Atlantic Avenue–Barclays Center to Borough Hall
• The IRT Lexington Avenue Line from Borough Hall to Grand Central–42nd Street
• The IRT 42nd Street Shuttle from Grand Central–42nd Street to Times Square
• The IRT Broadway–Seventh Avenue Line from Times Square to Van Cortlandt Park–242nd Street
• The IRT Lenox Avenue Line from 96th Street to 145th Street
• The IRT White Plains Road Line from 142nd Street Junction to 180th Street–Bronx Park

Planning for a rapid transit line in New York City started in 1894 with the enactment of the Rapid Transit Act. The plans were drawn up by a team of engineers led by William Barclay Parsons, chief engineer of the Rapid Transit Commission. The city government started construction on the first IRT subway in 1900, leasing it to the IRT for operation under Contracts 1 and 2. After the initial line was opened, several modifications and extensions were made in the 1900s and 1910s.

The Grand Central–42nd Street station (also signed as 42nd Street–Grand Central) is a major station complex of the New York City Subway. Located in Midtown Manhattan at 42nd Street between Madison and Lexington Avenues, it serves trains on the IRT Lexington Avenue Line, the IRT Flushing Line and the 42nd Street Shuttle. The complex is served by the 4, 6, and 7 trains at all times; the 5 and 42nd Street Shuttle (S) trains at all times except late nights; the <6> train during weekdays in the peak direction; and the <7> train during rush hours and early evenings in the peak direction.

The station is adjacent to Grand Central Terminal and Grand Central Madison, which serves all Metro-North Railroad lines east of the Hudson River and Long Island Rail Road lines east bound for another major hub Jamaica and several points east. There are multiple exits to Grand Central Terminal and to nearby buildings such as One Vanderbilt and the Chrysler Building. Numerous elevators make the station compliant with the Americans with Disabilities Act of 1990.

The IRT Broadway–Seventh Avenue Line (also known as the IRT Seventh Avenue Line or the IRT West Side Line) is a New York City Subway line. 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 Brooklyn Branch, known as the Wall and William Streets Branch during construction, from the main line at Chambers Street southeast through the Clark Street Tunnel to Borough Hall in Downtown Brooklyn, is also part of the Broadway–Seventh Avenue Line. The IRT Broadway–Seventh Avenue Line is the only line to have elevated stations in Manhattan, with two short stretches of elevated track at 125th Street and between Dyckman and 225th Streets.

The line was constructed in two main portions by the Interborough Rapid Transit Company (IRT), a private operator. The first portion, north of 42nd Street, was opened between 1904 and 1908, and is part of the first subway line in the city. The line ran from City Hall, up the Lexington Avenue Line, across 42nd Street, and up Seventh Avenue and Broadway, before splitting into the Broadway Branch and the Lenox Avenue Line. The second portion of the line, that south of 42nd Street, was constructed as part of the Dual Contracts, which were signed between the IRT, the New York Municipal Railway (a subsidiary of the Brooklyn Rapid Transit Company), and the City of New York. Among the various subway lines that were to be constructed as part of the contracts, the West Side Line was to be extended south along Seventh Avenue to serve Manhattan's West Side.

The Lexington Avenue/51st Street station is a New York City Subway station complex on the IRT Lexington Avenue Line and IND Queens Boulevard Line. The station is located on Lexington Avenue and stretches from 51st Street to 53rd Street in Midtown Manhattan. It is served by the 6 and E trains at all times; the F train on weekdays during the day; the <6> train during weekdays in the peak direction; the <F> train during rush hours in the reverse peak direction; and the 4 train during late nights.

The complex comprises two stations: 51st Street on the Lexington Avenue Line and Lexington Avenue–53rd Street (originally Lexington–Third Avenues) on the Queens Boulevard Line. Originally two separate stations, these were connected in 1988 via a transfer passage, which was opened with the construction of 599 Lexington Avenue. Approximately 50,000 riders transfer between the Lexington Avenue and Queens Boulevard Lines each weekday.

The 6 Lexington Avenue Local and <6> Pelham Bay Park Express are two rapid transit services in the A Division of the New York City Subway. Their route emblems, or "bullets", are colored forest green since they use the IRT Lexington Avenue Line in Manhattan. Local service is denoted by a (6) in a circular bullet, and express service is denoted by a <6> in a diamond-shaped bullet. On the R62A rolling stock, this is often indicated by LEDs around the service logo to indicate local or express service to riders. The LEDs illuminate in a green circle pattern for 6 local trains and in a red diamond pattern for <6> trains.

The 6 operates 24 hours daily between Pelham Bay Park in the Bronx and Brooklyn Bridge–City Hall in Lower Manhattan, making all stops in Manhattan. Additional service short turns at Parkchester in the Bronx during weekday rush hours and middays and does not operate to or from Pelham Bay Park; during this time, 6 trains that originate and terminate at Parkchester make all stops in the Bronx, while trains that serve the full route make express stops in the Bronx between Parkchester and Third Avenue–138th Street in the peak direction and are labeled as <6> Express trains; when the <6> Express is not running, 6 trains originate and terminate at Pelham Bay Park and make all stops in the Bronx.