The Northwestern Elevated had four tracks and could accommodate express trains in both directions throughout the day. This line presently has the only form of express service on the CTA, with the rush hour "Evanston Express" operation north of Belmont Avenue, and with Howard Red Line trains skipping certains stops served by the Ravenswood Brown Line. Four tracks remain in service north of Armitage, where Red Line trains emerge from the State Street Subway tunnels. Four tracks previously extended south of Armitage on the elevated route to Chicago Avenue, but use of the outside tracks was discontinued in 1961.
Until the 1950's, the Metropolitan West Side Elevated Railroad line had four tracks on its line between downtown and Marshfield, which was where the Douglas and the Logan Square and Humboldt Park lines split from the Garfield Park main line.
During periods of "A" and "B" operation, all trains were alternately "A" or "B" trains. "A" trains would stop at "A only" and "A and B" stops, while "B" trains would stop at "B only" and "A and B" stops. Express operation was thus possible without requiring additional express tracks.
There were two situations where lines branched into two segments, and "A" and "B" trains would alternate onto each branch. "A" trains went to the Englewood branch and "B" trains went to the Jackson Park branch, making all stops on their respective branches. And "A" trains went to the Congress line and "B" trains went to the Douglas line, making all stops on their respective lines. Trains still alternate between Englewood and Jackson Park on what now is the Green Line, making all stops. This practice also was continued on what now is the Blue Line until June 25, 2006, when the Douglas line became part of the new Pink Line.
On November 3, 1913, the elevated lines began allowing free transferring between each others' systems in the Loop. On that day, service on the Loop elevated was significantly restructured. The Lake Street and the Northwestern Elevated lines converted from left hand to right hand operation, and all trains began operating counterclockwise around the Loop. The South Side and the Metropolitan lines had used right hand operation from the beginning. In the Loop, the South Side and Northwestern Elevated trains began using the outer track, while the Lake Street and the Metropolitan trains would use the inner track.
During the 1940's and 1950's, Chicago's two subway lines were opened. By 1958, with just the Lake Street and certain former Northwestern Elevated lines (Ravenswood, Evanston Express) using the Loop elevated, Ravenswood and Evanston Express trains began using the inner track.
Effective September 28, 1969, with the opening of the Dan Ryan line and the linking of it to the Lake Street Line, bi-directional operation returned to the Loop. Right hand operation was adopted, with the Lake-Dan Ryan trains operating both directions via the Lake Street and Wabash Avenue legs only. Ravenswood trains would circle the Loop counterclockwise using the outer track, with Evanston Express trains circling clockwise using the inner track, operating during rush hours only. In addition, the "Loop-All Stations" service was introduced throughout the day, circling clockwise using the inner track. That service was discontinued September 30, 1977.
On February 21, 1993, the Lake Street Line was linked with the South Side Elevated lines, instead of with the Dan Ryan Line. On October 31, 1993, CTA opened its new rapid transit line to Midway Airport, with trains circling the Loop clockwise using the inner track. And on June 25, 2006, the Pink Line was created out of the former Douglas Line, circling the Loop clockwise using the inner track.
On April 2, 2007, Purple Line trains began operating counterclockwise using the outer track, sharing the track with the Brown Line. This was due to major construction projects at the Fullerton and Belmont stops, which would reduce the capacity of the north side lines. It thus became desirable for riders departing the Loop to be able to board either a Brown Line or Purple Line train on the same track, whichever arrived first. This practice ended December 29, 2008, with completion of that construction phase.
On February 21, 1993, the South Side elevated route was linked to the Lake Street elevated route, via the Loop elevated. Prior to then, the Dan Ryan and the Lake Street lines had been linked together. This was the beginning of the adoption of color names for the CTA lines. The Howard-Dan Ryan line became the Red Line, while the Lake Street and South Side elevated lines became the Green Line.
The Dan Ryan Line was opened on September 28, 1969, and was through routed with the Lake Street Line, using the Wabash Avenue and Lake Street legs of the Loop elevated. This lasted until February 21, 1993, when the Dan Ryan Line and the South Side elevated lines "traded places" with their through routes.