The oldest section of the line opened on May 31, 1900, running from the Loop to Wilson. It was constructed by the
Northwestern Elevated Railroad. The route was extended to
Central Street in
Evanston on May 16, 1908, via leased and electrified trackage belonging to the
Chicago, Milwaukee and St. Paul Railway. In November 1913, the North Side "L" was through routed with the South Side "L" lines through the Loop. The ground-level section between Leland Avenue and Howard Street was elevated on a concrete embankment structure and widened to four tracks in 1922. The State Street subway opened on October 17, 1943, providing two new tracks bypassing the Loop tracks and the portion of the North and South Side "L" lines not equipped with express tracks. By providing an express route free of the most restrictive curves on the
Chicago "L" and shorter than the old line it supplemented, the subway reduced running time by as much as eleven minutes for a one-way trip. The route was placed in operation on October 17, 1943, for a portion of the through north–south trains, although other trains continued to use the "L" both on through trips and on services circling the Loop and returning to the point of origin. In August 1949, the North–South route was revised to create a more efficient routing through the
Loop and handle the heavy volumes of passenger traffic using it. The Howard branch was paired with the Englewood and Jackson Park branches through the State Street subway, using the 13th Street portal and the other lines routed to the Loop "L". The
Dan Ryan Branch opened in September 1969, and was paired with the Lake Street branch to form the
West-South route, almost universally referred to as the Lake–Dan Ryan route. It operated over the Lake Street and Wabash Avenue sides of the Union Loop. This "interim" service was created mainly for the purpose of providing through service between the West Side and the South Side in anticipation of the 1968
Loop Subway Project. When the controversial
subway project was cancelled in 1979, the Lake-Dan Ryan service remained and lasted for 24 years. For much of the 20th century, the
Howard–Englewood/Jackson Park route was equally compatible in terms of passenger service until the late 1960s through the 1970s. However, passenger traffic volumes began to shift on the South Side lines, with more riders using the newer Dan Ryan Line (which runs four miles (6 km) further south) and fewer riders on the older "L" lines. This level of ridership allowed the CTA to develop a more efficient system by combining the more heavily used rail lines together and combining the other lines together, providing increased service capacity for the routes that need it. The Red Line was created in 1993, when the CTA adopted color-coded system for all of its "L" routes. In February 1993, a new connection opened from the State Street Subway south of Roosevelt to Cermak–Chinatown. The Howard branch was paired with the Dan Ryan branch to form the current Red Line, while the Lake Street branch was paired with the Englewood and Jackson Park branches to form the Green Line. A further operational benefit of this switch was that this freed up capacity in the Loop needed for the addition of the
Orange Line to
Midway Airport. The former tracks used to move West-South trains from the Loop to the Dan Ryan line are still retained for non-revenue moves and service disruptions. The
Dan Ryan Branch underwent a rehabilitation period to improve its aging infrastructure which ended in early 2007. This work included upgrading the power and signal systems, and rehabilitating the stations with improved lighting, a cleaner appearance, and new escalators and elevators. The CTA has plans to expand Red Line service to ten car trains from the current eight-car trains.
Red Ahead rehabilitation In spring 2012, the CTA started a station and track rehabilitation program dubbed "Red Ahead", beginning on the North Side Main Line, which is called the "Red North" project. The program monitors the full route of the Red Line, excluding the Loyola, Bryn Mawr, Sheridan, or Wilson station. This also does not include stations between Wilson through Fullerton, nor the State Street subway. In May 2012, the CTA started to work on the North Side Main Line stations of the Red Line which includes Jarvis, Morse, Granville, Thorndale, Berwyn, Argyle and Lawrence. The stations are listed in order, starting at Granville, then Morse, Thorndale, Argyle, Berwyn, Lawrence and finally Jarvis. This project started in June 2012 and was completed in December 2012. This project is also part of the Red Ahead's "Red & Purple Modernization" Project. The Red and Purple Modernization Project included a redesign of a
diamond junction north of Belmont Station into a
flyover for Brown Line trains. This project was expected to decrease train backups and increase the number of trains that can cross the junction per hour. The project was criticized by 2015 mayoral candidate
Chuy García and local residents in the
Lakeview neighborhood who organized a referendum to stop it. The
Federal Transit Administration passed the CTA's environmental review on the flyover in January 2016 and received a $1.1 billion federal grant the following year. 16 properties affected by the flyover were demolished. Construction began on October 2, 2019, and the flyover entered service on November 19, 2021. ==Upcoming extension==