The Blue Line is a Chicago "L" line which runs from O'Hare International Airport at the far northwest end of the city, through downtown via the Milwaukee–Dearborn subway and across the West Side to its southwest end in Forest Park, with a total of 33 stations (11 on the Forest Park branch, 9 in the Milwaukee–Dearborn subway and 13 on the O'Hare branch). It is the longest line on the Chicago "L" system and second busiest, and one of the longest local subway/elevated lines in the world. It is operated by the Chicago Transit Authority and had an average of 72,475 passengers boarding each weekday in 2023.
Before the adoption of color-coded names in 1993, the Blue Line was referred to as the West-Northwest Route, or more commonly, the O'Hare-Congress-Douglas route for its three branches. The Congress and Douglas branches were renamed for their terminals, Forest Park and 54th/Cermak. Blue Line service on the Douglas branch was replaced in April 2008 by the Pink Line. The Blue Line is one of five "L" lines that run into Chicago suburbs, with the others being the Green, Purple, Pink, and Yellow lines. The Blue Line runs in three suburbs: Rosemont, Oak Park, and Forest Park. The Blue Line also has two in-system transfers (both in the Loop), and does not share tracks with any other 'L' line. It is also the only line in Chicago with more than one station having the exact same name: two stations at Harlem Avenue and two stations on Western Avenue.
The Blue Line and Red Line offer 24-hour service every day; the Orange Line will begin 24/7 service in 2026.
Route
O'Hare branch The O'Hare branch is the longest section of the Blue Line () and comprises both the oldest and newest segments of the entire route. The line starts at O'Hare International Airport in an underground station below the airport's main parking garage, with direct pedestrian access to Terminals 1, 2 and 3. The line emerges in the median of the O'Hare main access road (Interstate 190) just northwest of Terminal 5, about a mile (1.6 km) west of Mannheim Road.
The line runs in the median of Interstate 190 east through Rosemont. The line has a station at River Road in , which is also the location of the northern storage yard and served as a temporary terminal from 1983 to 1984 while the O'Hare station was being completed. The tracks then tunnel beneath the Kennedy Expressway/Northwest Tollway interchange near the Des Plaines River.
The line runs in the median of the Kennedy Expressway (Interstate 90) until a point southeast of Addison Street, making stops at , , , , and . Between Montrose and Irving Park, the line tunnels beneath an express lane exit. South of Addison Street, the line descends into a subway and turns south under Kimball Avenue. The line travels under Kimball Avenue and Milwaukee Avenue through Logan Square, making stops at and .
South of Logan Square, the line emerges above ground onto an elevated structure parallel to Milwaukee Avenue. This section of structure, built in 1895 as part of the Metropolitan Elevated's Logan Square branch, is the oldest portion of the Blue Line, and the sole section of the line on an elevated viaduct. The three stations on this section (, , and ) are also the only three stations on the line to use side platforms instead of island platforms.
Milwaukee–Dearborn subway
At the intersection of Ashland and Milwaukee Avenues, the Blue Line descends underground, swings over to Milwaukee Avenue, and continues southeast towards downtown under Milwaukee Avenue (with stops at , and ). The line then turns east under Lake Street, crossing beneath the Chicago River, and makes a stop at , where in-system transfers are provided to 'L' trains on the Loop.
East of Clark/Lake, the tracks swing south under Dearborn Street, with a continuous platform similar to the continuous platform used in the adjacent State Street subway, with stops at , , and .
South of Jackson, the line turns west under Ida B. Wells Drive (with stops at and , which is two blocks south of Union Station). The tracks then emerge from a portal near in the median of the Eisenhower Expressway (Interstate 290) and continue west.
Forest Park branch
After exiting the subway, the tracks continue west in the median of the Eisenhower Expressway as the Forest Park branch, formerly called the Congress branch (though it is still internally referred to as such), the successor to the Garfield Park Branch.
Immediately west of , the Forest Park branch tracks diverge to permit the Loomis ramp up to the Douglas Branch elevated structure. The ramp was formerly used in revenue service from 1958 to 2008 when the Blue Line operated over the Douglas branch and the Forest Park branch.
With the replacement of Douglas branch Blue Line service with the Pink Line, the ramp is now non-revenue trackage used for the transfer of 5000 series trains that the Blue Line borrows from the Pink Line for weekday rush hour service. The ramp is also used for non-revenue equipment movement and access to the Skokie Shops, as it's the only link between the Blue Line and the rest of the "L" system. The Forest Park branch remains in the median of the expressway through the west side of Chicago until it reaches a portal at Lotus Avenue.
At this point, the tracks pass beneath the eastbound expressway lanes before emerging on the south side of the expressway next to the CSX Transportation (Baltimore and Ohio Chicago Terminal Railroad) tracks. The route passes through Oak Park and into Forest Park. In the vicinity of Desplaines Avenue the tracks rise and make an S-curve north over the expressway before terminating at the station.
The Forest Park branch has historically had depressed ridership due to competing for riders with both the Green and Pink Lines and running in a highway median, which discourages ridership because the air and noise pollution from passing vehicles discourages the construction of dense housing nearby and makes it unpleasant to get to and wait at the stations. But it has also suffered due to neglect of the tracks past Illinois Medical District station, with most of the infrastructure being untouched since its construction in 1958. Due to deferred maintenance and poor geotechnical engineering (e.g. drainage issues), this track has rapidly deteriorated since the 2010s, with subsequent speed restrictions further reducing ridership. In 2016, just 13.1% of the Forest Park branch was a slow zone, with some sections having official speed limits as low as but running at a maximum of in practice; by 2020, it had increased to 61.6%; by 2024, it increased again to 72.4%; and in 2026, slow zones accounted for 84.4% of the branch.
Between 2014 and 2024, the proliferation and increasing severity of slow zones on the Forest Park branch increased travel times by 10 minutes. Though phase 1 of the Forest Park Branch Rebuild (which rebuilt tracks and stations between LaSalle and Illinois Medical District stations) was completed in 2023 and a federal grant to rebuild two additional miles of track (between Kedzie and Pulaski stations) in 2027 was awarded to the CTA in 2024, the next 3 phases of the project to rebuild the rest of the Forest Park branch have yet to be announced.
Unfortunately for Forest Park branch riders, the CTA has no major plans for addressing most of the branch's deficiencies unless it can receive federal funds to redo the line as part of IDOT reconstructing the Eisenhower Expressway, which the CTA has been holding out for as the Eisenhower has reached the end of its useful life. Because no major work was planned for the Eisenhower in IDOT's 2022-2027 capital plan, it is unlikely any such funds will become available to improve the Forest Park branch until at least 2027.
Operating hours and headways
Like the Red Line, the Blue Line runs 24 hours a day. Most trains run between O'Hare and Forest Park stations, but others terminate at a different station along the line. On weekdays, service runs very frequently (10–20 tph (trains per hour)) during rush hour (with some trains running short turn services between UIC-Halsted and Jefferson Park or Rosemont), and 6–8 tph during the midday and nighttime.
On Saturdays, service runs 8 tph in the early morning, then increase to 10 tph during the day, then 8 tph at night. On Sundays, service runs 6–8 tph early morning, then increase to 10 tph all day, then 6 tph at night. On the weekends, every other Blue Line train operates between O'Hare and UIC-Halsted only during the daytime, doubling the headways up to 5 tph. Between approximately midnight and 5:30 a.m., night owl service on the Blue Line ranges between 3–4 tph.
Rolling stock
The Blue Line is operated with 2600-series, 3200-series, and 7000-series railcars. In 2018, some of the 2600-series cars from the Blue Line fleet were replaced with rehabbed 3200-series cars from the Brown and Orange Lines, with some of the 2600-series cars being reassigned to the Orange Line to replace them. These cars entered service on the Blue Line on September 17, 2018. During rush hours, the Blue Line additionally borrows two 5000-series trainsets from the Pink Line for short-turn trips between UIC-Halsted and Jefferson Park, although these cars remain officially assigned to the Pink Line and are operated by Pink Line operators.
History
The Blue Line is the successor to the Metropolitan West Side Elevated Railroad, which built a series of 'L' lines servicing the West Side of Chicago beginning in 1895. The first section to be built by the Metropolitan extended west in the vicinity of Van Buren Street from an independent terminal at Canal and Jackson Streets to Marshfield Avenue, and then northward in the vicinity of Paulina Street to Damen and Milwaukee Avenues. Service on this section began on May 6, 1895. The structure was completed from Damen Avenue to Logan Square on May 25, 1895.
The next stage in the development of the West Side 'L' came on June 19, 1895, when the Garfield Park Branch was added, extending west in the vicinity of Van Buren Street and Harrison Street from Marshfield Avenue to Cicero Avenue. An extension of service over the tracks of the Aurora, Elgin and Chicago Railroad to a new terminal at Desplaines Avenue was established on March 11, 1905. A subsequent extension to Westchester opened on October 1, 1926, over tracks that had originally been built by the CA&E with the intentions of building a bypass route. (Low ridership would prompt the CTA to discontinue service on the Westchester extension on December 9, 1951.)
Another branch line was added to the rapidly growing Metropolitan on July 29, 1895, when trains began operating over the Humboldt Park Branch, splitting off from the Logan Square Branch at and running west alongside North Avenue to a terminal at Lawndale Avenue. This was followed by still another addition when the Douglas Park Branch was placed in operation as far south as 18th Street on April 28, 1896.
As the southwest area of the city developed, the Douglas Park Branch was extended from 18th Street to Western Avenue in September 1896; to Pulaski Road in June 1902; to Cicero Avenue in December 1907; to Central Avenue in August 1912; to 62nd Avenue in August 1915, and to Oak Park Avenue in Berwyn on March 16, 1924. The Douglas Park branch was later cut back to 54th Avenue in Cicero.
The Metropolitan West Side Elevated began service onto the Loop on October 11, 1897, and a rush period stub terminal at Wells Street was added October 3, 1904. For much of the early 20th century and through the 1940s, service on the West Side Elevated lines went unchanged until 1947, when the Chicago Transit Authority took control of the 'L', initiating a series of massive service cuts and station closings (that would last until the 1980s).