Blue Line (CTA)

The Blue Line, also known as the O'Hare-Congress Line and the West-Northwest Line, is a long Chicago "L" line which goes through the Loop 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 at Forest Park, with a total of 33 stations. It is the CTA's second busiest rail line, with an average of 186,796 passengers boarding each weekday in September 2012.
Background.
The Blue Line and Red Line are the only two routes of the Chicago "L" system to currently run 24 hours a day and is one of only six rapid transit rail systems in the United States to do so (the others being the PATCO Speedline, Staten Island Railway, the PATH lines, Philadelphia's SEPTA and the New York City Subway). 
The Blue Line is also one of only two lines with more than one station with the same name, with the Green Line being the other (It has two stations at Harlem Avenue: one in the Kennedy Expressway on the Northwest side and one on the south side of the Eisenhower Expressway in Forest Park, Illinois. It also has two stations on Western Avenue: one on the O'Hare branch and one on the Congress branch). The Blue Line also has only three in-system transfers (all in the Loop), contains a combination of both the oldest and newest portions of 'L' tracks, and does not share tracks with any other 'L' line.
Code name.
Before the adoption of color-coded names, the Blue Line was referred to as the West-Northwest Route (which it is still sometimes referred to as today) 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, when the current color naming system was adopted in 1993. Blue Line service on the Douglas Branch was discontinued in April 2008 and replaced by the Pink Line.
Trivia.
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, Green, and Purple Lines are the only three rail lines that run through more than one suburb, with the Green and Purple Lines running through two, but the Blue Line actually runs through three, making it the rail line that runs through the most suburbs on the Chicago 'L' system.
In March 2014, a Blue Line train was derailed and crashed at the O'Hare train station, injuring 32 people.