The original Logan Square branch was separated into several sections in 1951, some of which remain in revenue service as of 2023[update].

The publicly-owned Chicago Transit Authority (CTA), which had assumed control of the "L" in 1947, decided to instead discontinue service on the elevated lines with the opening of the subway.

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.

[14] Chicago petitioned the Public Works Administration (PWA) for construction funds for a subway on State Street in 1937.

[16] Harold L. Ickes, the administrator of the PWA and a longtime Chicagoan, vetoed the streetcar tunnel plan and insisted instead on a second subway that would go under Dearborn Street and Milwaukee Avenue, which would provide a more direct route from Logan Square to downtown.

[16] Although this idea engendered considerable local opposition, especially from mayor Edward Joseph Kelly, Ickes's influence in the federal government led to the Dearborn plan being adopted in 1938.

[16] A 1939 plan also introduced the idea of replacing the Metropolitan's main line and Garfield Park branch with a section of rapid transit operating through a proposed superhighway on Congress Street (the eventual Interstate 290).

[19] Damen Tower serving the Humboldt Park branch divergence was rebuilt with the expectation that it also would switch trains between the subway and the elevated, much like the State Street subway connects with the earlier elevated North Side main line that remained standing after its construction,[20] and as late as 1949 commuters were promised such a setup that would have preserved the old Logan Square trackage.

[22] Since construction had not started on the Congress Line, trains in the Dearborn subway stopped at its southern terminus at LaSalle and turned back.

[22] Despite its incomplete state, and complaints from riders no longer given a direct trip to the Near West Side,[24][25] the new subway had over 60 percent higher ridership than the old Logan Square branch by the end of the year.

The surviving portion of the Logan Square branch was extended to Jefferson Park in 1970, Rosemont in 1983, and O'Hare in 1984.

The portion between the junction northwest of the subway portal and the other Metropolitan West Side branches was retained as the Paulina Connector, a non-revenue connecting track, as the other Met branches were rerouted from the Loop into the south end of the Milwaukee–Dearborn subway, meaning there was no other connection to the rest of the "L" system.

[31] When the first 6000-series were delivered in August 1950, they were placed on the Logan Square and Humboldt Park branches for trials in anticipation of the subway's opening.

[6] Plans to eliminate owl service on the branch had been raised by early 1950, but was deferred until the opening of the subway.

[35] At the same time, a weekly pass was introduced, the first in a major American city, for $1.25 ($20.24 in 2021) for rides outside of Evanston and Wilmette.

[7] As late as 1960, after the original Logan Square branch was supplanted, there was no fare control at Damen besides the station agent.

[36] Originally, station agents were on duty 24 hours a day; conductors were introduced on the Logan Square, Humboldt Park, and Northwest branches to instead collect fares on trains during night and off-peak hours in 1931 and remained in use through 1937, although the Northwest branch regained 24-hour station agents in 1935.