The line mostly operated concurrently with the Chicago Aurora and Elgin Railroad (CA&E), who started sharing tracks with the Met Elevated in 1902.

In the early years, the line expanded greatly, following the route taken by the CA&E; while the Garfield originally only extended to 48th Avenue, the line was soon extended to 52nd Ave/Laramie, and finally to Des Plaines Avenue; the Westchester branch was added in 1926, but it was really just a transfer of local service between CA&E and the Chicago Rapid Transit Company.

A set of temporary tracks were placed slightly north of the right-of-way to allow for construction to continue on the Congress Expressway and the construction of temporary stations at Des Plaines Avenue and Ridgeland Ave. By 1955, while the Garfield and Met Main were still a major part of the "L" system, the vast majority of the stations on the line had already been closed and demolished; the Westchester branch was abandoned 4 years earlier, ending CTA train service west of Des Plaines.

Initially, some of the previously closed stations were served by the CA&E after CTA ended service, but the interurban, already financially bleeding due to the rise of the automobile, ended service to downtown Chicago on September 20, 1953 due to the new temporary track routing - terminating at Des Plaines, where a CTA transfer was required to continue further downtown.

The brand new Congress Branch opened on June 22, 1958 and connected to the Milwaukee–Dearborn subway, replacing and closing down the last remnants of the old Garfield Park and Metropolitan Main Line.

After being somewhat split apart into several different lines due to construction, the remnants and successors of the Metropolitan West Side Elevated Railroad were combined into one line once again as the Logan Square branch, the oldest part of the Met, was routed to downtown through the Milwaukee–Dearborn subway, splitting off into the Congress and Douglas branches depending on train.