The Pink Line is the CTA's newest line and began operating on June 25, 2006 running entirely on existing tracks. It took over the Douglas Branch of the Blue Line (which provided supplemental rush hours service until April 25, 2008) and routed these trains to the loop via the then unused in revenue service Paulina Connector to reach the Loop via the Lake Branch of the Green Line. Trains run via the inner loop to reverse before returning to 54/Cermak. The Pink Line was created to give riders on the Cermak branch of the Blue Line more frequent service downtown and streamline operations on the Blue Line which in turn gave Forest Park riders better service.

The line serves the Douglas Branch of the Metropolitan Elevated which opened between 1896 and 1912. Trains on the branch originally reached the loop via the Main Line of the Metropolitan Elevated that follows the same corridor as today's Blue Line in the median of the Eisenhower Expressway. In 1954 it was closed to allow the Expressway to be finished. Trains to the Douglas Branch were rerouted to the Lake Street elevated until 1958, exactly replicating today's Pink Line. In 1958 the expressway was finished and the Douglas Branch began running as one of two branches (the other being Forest Park) of the West-Northwest Route (today's Blue Line) that ran for 50 years. The Paulina Connector to the Lake Street 'L' was kept for non-revenue track moves because it is the Blue Line's only track connection to the rest of the 'L' System.

In the 1990s the Douglas branch (then renamed as today, the Cermak branch) of the Blue Line saw some of the worst service cuts on the CTA with all weekend service on the branch discontinued in 1998. It also had an extremely high number of slow orders. In October 2001 the CTA announced 'Renew the Blue' to bring the branch back into a state of good repair. In terms of stations all but 3 were completely rebuilt on the branch and only one, Kedzie, was given a historic renovation and left with its historic station house. These rebuilt stations all have island platforms with unique and modern mesh silver posts and canopies that are triangular and a mixture of metal and glass allowing natural light to shine through below. Every stop was also given a mostly decorative compass outside the station entrance. Weekend service was restored on January 1, 2005 (it would have been restored sooner if construction work wasn't ongoing) by rerouting half of the Forest Park Blue Line back trains to 54th and Cermak.