The main line and its associated branches were originally operated by the Metropolitan West Side Elevated Railroad, one of four companies that built what would become the Chicago "L".

The Metropolitan West Side Elevated Railroad Company was granted a 50-year franchise by the Chicago City Council on April 7, 1892,[1] and began securing right of way shortly thereafter.

After 1914, the Metropolitan purchased electricity from Commonwealth Edison, but the Loomis shop would remain standing until the construction of the Congress street expressway.

[14] The Loop would prove to be frequently over capacity, and the Metropolitan was forced to turn some downtown-bound trains back at Canal starting in 1900, constructing a special platform extension and track stub for the purpose.

The Aurora Elgin & Chicago Railway (AE&C) began to use the Garfield Park branch and main line to access the Fifth Avenue terminal on March 11.

[18] The publicly owned CTA replaced the CRT, which had proven chronically unprofitable, in 1947, having been chartered for the purpose by the Illinois General Assembly in 1945.

[20] The main line was an extremely busy place in the late 1940s and early 1950s; in 1948, several trains on each track were within eyesight of one another during rush hours.

Arising and benefiting from the poor road conditions in Chicago's western suburbs, the service carried an average of 22 trains a week by October 1907.

The service declined and was discontinued in the 1930s, but the Chicago Transit Authority (CTA)'s charter continues to allow it to run funeral trains.

The sharpness of the curves of the outer tracks' bowing out to accommodate the platforms, however, was considered dangerous and some of the stations were reconfigured between 1898 and 1914.

[30] The Old Chicago Main Post Office was greatly expanded in 1932; built in the way of the proposed superhighway, it was constructed with a hole in its bottom to allow for traffic to run through it, which would be used in when the expressway was completed.

In the final settlement of those negotiations, the City assumed responsibility for the expressway and rapid transit facilities in between the Chicago River and Laramie Avenue; in exchange, the CTA granted the City and Cook County a permanent easement on the right-of-way of the Metropolitan main line and Garfield Park branch between Desplaines Street and Sacramento Boulevard to construct the facilities, and agreed to demolish the elevated structures at its own expense.

[17][27] Starting on September 20, 1953, the Garfield Line was rerouted onto temporary street-level trackage between Sacramento and Aberdeen Avenue.

[17] The Congress branch was noted as the world's first rapid transit in the median of an expressway, a claim that it disputes with the Cahuenga Parkway in Los Angeles.

The Congress branch's time between Forest Park and downtown was 17 minutes faster than the equivalent distance on the old Garfield Line.

[34] On the Congress branch, Halsted (now UIC-Halsted)[28] and Racine were explicitly designed to replace their respective stations on the Garfield Line.

[38] In addition, Illinois Medical District replaced three stations on the Garfield Line, including Marshfield,[39] and Racine has an entrance on Loomis Street one block east of Laflin.