The Lake Street Elevated, also known as the Lake branch, is a 8.75 mi (14.08 km) long branch of the Chicago "L" which is located west of the Chicago Loop and serves the Green Line for its entire length, as well as the Pink Line east of Ashland Avenue.

When the completed Loop opened October 3, 1897, the Lake Street Elevated became the first line to utilize the entire quadrangle.

The Randolph and Cuyler branches were part of trackage rights given by the Chicago, Harlem & Batavia Railway, its trains continued east to Grand Central Station.

On May 20, 1910, service was extended to its terminal approximately two blocks west of Harlem Avenue in Forest Park.

This new system decreased the travel time of 24–35 minutes, which was considered by the CTA and its users to be a great success.

[12][13][14] Until 1969, the Lake Branch was an independent branch line providing service in one direction only, like the Ravenswood and Evanston Express Lines: from the Loop to the West Side, Oak Park, and Forest Park.

On September 28, 1969, it was paired with the newly opened Dan Ryan branch to form the Lake–Dan Ryan Line, also called the West-South Route, via the north and east legs of the Loop, the South Side Elevated, and the connection along 18th Street.

On January 9, 1994, the Lake Street Elevated and the entire Green Line closed for two years for a rehabilitation project.