When the first subway opened between 1904 and 1908, one of the main service patterns was the West Side Branch, which the modern 1 train uses. Trains ran from Lower Manhattan to Van Cortlandt Park via what is now the IRT Lexington Avenue Line, 42nd Street Shuttle, and IRT Broadway–Seventh Avenue Line. There was both local and express service with express trains using the express tracks south of 96th Street. Some express trains ran to Atlantic Avenue in Brooklyn via the Joralemon Street Tunnel during rush hours while all other trains turned around at City Hall or the South Ferry outer loop.
On June 3, 1917, the first portion of the Broadway–Seventh Avenue Line south of Times Square–42nd Street, a shuttle to 34th Street–Penn Station, opened; a separate shuttle service to 34th Street was placed into service. This shuttle was extended south to South Ferry, with a shorter shuttle on the Brooklyn branch between Chambers Street and Wall Street, on July 1, 1918. Finally, 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.
As of 1934, all 1 Express trains were running from 242nd Street to New Lots Avenue weekdays and Saturdays during the day, alternating to New Lots Avenue and Flatbush Avenues evenings and Sunday afternoons, and were split at Brooklyn Museum on Sunday morning with the south half going to New Lots Avenue and the north half to Flatbush Avenue. Late night service was not operated. All 1 Local trains ran from 137th Street (extended to Dyckman Street during rush hours) to South Ferry days and evenings, and 242nd Street to between New Lots and Flatbush Avenues late nights.
On September 5, 1937, the practice of splitting Sunday morning trains at Brooklyn Museum was discontinued, with the alternate trains going to New Lots Avenue or Flatbush Avenue.
As of July 1, 1938, all evening and Sunday trains were rerouted to New Lots Avenue.
By 1945, all 1 Local peak period trains were cut back from Dyckman Street to 137th Street.
Beginning on May 10, 1946, all 1 Brooklyn trains were made express during late nights running on 12 minute headways. Previously all 1 trains ran local from 12:30 to 5:30am and they alternated between Flatbush and New Lots Avenues. On December 20, 1946, all late night trains were routed to Flatbush Avenue, while Sunday service still alternated between Flatbush and New Lots Avenues. On June 12, 1949, 137th Street to South Ferry Sunday locals were discontinued, but were resumed on March 5, 1950, at which time Sunday service was also rerouted to New Lots Avenue. On March 15, 1954, weekend 137th Street to South Ferry locals were discontinued, and simultaneously weekend Brooklyn trains were rerouted to Flatbush Avenue.
An attempt was made to extend express service further north on January 14, 1955, when alternate rush trains ran express between 137th and 96th Streets in the peak direction. This proved unsuccessful, and ended on June 28, 1956. Meanwhile, in Brooklyn, weekday trains were rerouted to Flatbush Avenue on December 20, 1957, and evening 137th Street to South Ferry locals were discontinued on May 23, 1958.
Under a $100,000,000 rebuilding program, increased and lengthened service was implemented during peak hours on the 1 train. Switching north of 96th Street was eliminated. On February 6, 1959, 1 trains began to run between 242nd Street and South Ferry all times. Trains began to be branded as Hi-Speed Locals, being as fast as the old express service was with new R21s and R22s on the line. During rush hour in the peak direction, alternate trains, those running from 242nd Street, made no stops except 168th Street between Dyckman and 137th Streets in the direction of heavy traffic. The bypassed stations were served by locals originating from Dyckman Street.
PM rush local/express service was discontinued on February 2, 1959, and morning rush express service was revised on January 8, 1962 to running non stop from 225th to Dyckman Streets and 168th to 137th Streets. This express service was discontinued on May 24, 1976, after which all 1 trains began to make all stops.
On August 21, 1989, the 1/9 weekday skip-stop service started. The plan was to have skip-stop service begin north of 116th Street–Columbia University, but due to criticism, most notably that riders did not want 125th Street to be a skip-stop station, skip-stop service operated north of 137th Street–City College between the hours of 6:30 am and 7:00 pm. All 1 trains skipped Marble Hill–225th, 207th, 191st and 145th Streets, while all 9 trains skipped 238th, 215th, Dyckman and 157th Streets. On September 4, 1994, midday skip-stop service was discontinued, and 191st Street was no longer a skip-stop station.
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. It ran only between 242nd Street and 14th Street, making local stops north of and express stops south of 96th Street. The skip-stop service with the 9 train was suspended. On September 19, after a few switching delays at 96th Street, service was changed. All 1 trains made all stops from 242nd Street to New Lots Avenue via the Clark Street Tunnel and IRT Eastern Parkway Line, to replace all 3 trains (which terminated at 14th Street) at all times except late nights, when it terminated at Chambers Street in Manhattan instead. On September 15, 2002, all 1 trains returned to the South Ferry Loop and 9 skip-stop service was reinstated. But Cortlandt Street, which was directly underneath the World Trade Center, was demolished as part of the clean-up and will be rebuilt as part of the World Trade Center Transportation Hub.
The last 9 train operated on May 27, 2005, and 9 train service was discontinued on May 31, 2005, and all 1 trains now made all stops. The skip-stop service made less sense by 2005 because of the increased number of trains being run and the higher ridership at the bypassed stations; the MTA estimated that eliminating skip-stop service only added 2 1⁄2 to 3 minutes of travel time (for passengers at the northernmost stations at 242nd Street and 238th Street) but many passengers would see trains frequencies double, resulting in decreased overall travel time (because of less time waiting for trains).
In October 2008, the MTA agreed to pay the Port Authority of New York and New Jersey (PANYNJ) to rebuild Cortlandt Street station directly under Greenwich Street and the 9/11 Memorial as part of the World Trade Center Transportation Hub. The planned opening of the station has been delayed from 2014 to 2018. The tracks are walled off for the protection of the workers while the construction is underway. As a result of the massive excavation in the World Trade Center site, the 1 train uses an enclosed elevated structure briefly when passing the site of the station. The station will have a connection to the World Trade Center Transportation Hub and Fulton Street when it reopens.
On March 16, 2009, the new South Ferry station opened, replacing the original loop station. However, 1 service was affected by Hurricane Sandy in October 2012, following serious flood damage at South Ferry. Rector Street served as a temporary terminal until April 4, 2013, when the 1 returned to the reopened loop station, also serving as a temporary terminal until the new South Ferry Station opens again in 2017.
The 1 uses the following lines with the same service pattern at all times.