The 1977 Chicago Loop derailment occurred on February 4, 1977, when a Chicago Transit Authority elevated train rear-ended another on the northeast corner of the Loop at Wabash Avenue and Lake Street during the evening rush hour. The collision forced the first four cars of the rear train off the elevated tracks, killing 11 people and injuring at least 268 as the cars fell onto the street below.

The Chicago Transit Authority (CTA) is the operator of mass transit in Chicago, Illinois, and some of its surrounding suburbs, including the trains of the Chicago 'L' and CTA bus service.

The CTA is an Illinois independent governmental agency that started operations on October 1, 1947, upon the purchase and combination of the transportation assets of the Chicago Rapid Transit Company and the Chicago Surface Lines streetcar system. In 1952, CTA purchased the assets of the Chicago Motor Coach Company, which was under the control of Yellow Cab Company founder John D. Hertz, resulting in a fully unified system. Today, the CTA is one of the three service boards financially supported by the Regional Transportation Authority and CTA service connects with the commuter rail Metra, and suburban bus and paratransit service, Pace.