The Blue Line is a 26.93-mile-long (43.34 km) 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).

The Congress and Douglas branches were renamed for their terminals, Forest Park and 54th/Cermak, when the current color naming system was adopted in 1993.

The O'Hare branch is the longest section of the Blue Line (14.6 miles (23.5 km)) and comprises both the oldest and newest segments of the entire route.

The line then turns east under Lake Street, crossing beneath the Chicago River, and makes a stop at Clark/Lake, where in-system transfers are provided to 'L' trains on the Loop.

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.

In the vicinity of Desplaines Avenue the tracks rise and make an S-curve north over the expressway before terminating at the Forest Park station.

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.

[6] 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.

[citation needed] The Metropolitan lines began to be reshaped into the current Blue Line on February 25, 1951, when the CTA opened the Milwaukee-Dearborn Subway, connecting the Logan Square Branch with the Loop on a fast, efficient and more direct routing to downtown, rather than the previous circuitous route that saw these trains entering the Loop at the southwest corner.

[11] With opening of the Dearborn Subway, the old elevated alignment between Evergreen Portal and Marshfield Junction was decommissioned, used only for moving out-of-service rail cars.

The Humboldt Park Branch was cut back to a full-time shuttle between Damen and Lawndale, and discontinued a year later on May 3, 1952.

The Garfield Park elevated was replaced by the Congress line on June 22, 1958,[12] pioneering the world's first use of rail rapid transit and a multi-lane automobile expressway in the same grade-separated right-of-way.

The new line connected with the Milwaukee-Dearborn Subway at the Chicago River and extended westward to Des Plaines Avenue in Forest Park.

[14] A five-mile (8 km) extension of the route via a short subway connection and the Kennedy Expressway median from Logan Square to Jefferson Park opened on February 1, 1970.

Just before Logan Square, trains diverted off of the old elevated structure and entered the subway under Milwaukee and Kedzie Avenues to a portal just south of Addison Street, then emerged in the median of the Kennedy Expressway to the temporary terminal at Jefferson Park.

The increased ridership that resulted from the extension prompted the CTA to build the second phase of the project, and extend the line the rest of the way to O'Hare.

On October 15, 2015, the CTA announced the completed installation of 4G wireless service on the Blue Line in between the Logan Square and Belmont stations.

In the future this will mean that the CTA will be the largest rapid transit system with 4G coverage in subway tunnels and stations, this is targeted for the end of 2015.

[19] The work was officially completed on January 8, 2005, with new elevated structures, tracks, rebuilt stations, new communication networks and an upgraded power system along the route.

These changes were scheduled to be implemented for a 180-day trial period beginning June 25, 2006, and after their evaluation in early 2007, the Pink Line remained in service.

Beginning April 28, 2008, the CTA began a six-month experimental ceasing of Blue Line operations on the Douglas.

However, in 2008, the Regional Transit Authority revealed a plan to the RTA board to expand commuter rail and bus service, which included a 13.3-mile (21.4 km) extension of the Blue Line, from its current western terminus at Forest Park to Yorktown Center in Lombard, Illinois.

It was intended that the interurban Chicago Aurora and Elgin Railroad, which had utilized the Garfield Park Elevated until 1953 to reach its Loop terminal at Wells Street, would use these extra tracks.

Between Grand/Milwaukee and Clark/Lake in the Milwaukee-Dearborn Subway, two more stub tunnels exist, continuing west under Lake Street while the in-service tracks turn northwest under Milwaukee Avenue.