Most of the early "L" lines had some form of express/local operation, although the Northwestern Elevated was the only company to have a four-track main line with completely separated express/local operations. As the population of the South Side continued to rise in the early 20th century, the South Side Elevated decided that an express track needed to be added between 12th Street and 43rd Street to serve the expanding ridership as well as the new branches planned for Englewood, Kenwood and the Stock Yards complex.

The addition of a third track was difficult due to the narrow right-of-way that the South Side operated in. Demolition of adjacent buildings was quickly ruled out as too costly so it was decided to build over the adjacent alleys. But permission from the city to do this came with a high price. A seemingly innocuous clause stating the company was to "keep its said right of way at all times free and unimpeded and open for public travel, except by its columns, stairways, and appurtenances requisite and convenient for its elevated railroad and the proper care and repair thereof," really meant that the "L" had to demolish all of its street-level station houses and replace them with mezzanine facilities so traffic could pass beneath.

The third track was added in an alley adjacent to the east of the existing structure, meaning that the northbound platforms from 18th to 39th Streets and the southbound platforms at 12th Street and Indiana Avenue had to be removed and replaced. All of the intermediate stations (except 12th Street) and surrounding track structure had to be graded up to 1.44% to provide sufficient clearance.

In March 1907, work was completed and express operations began. Express trains ran nonstop from 43rd Street to 12th Street (Roosevelt Road) northbound in the morning and southbound in the evening, beginning March 26. Express trains were able to reach speeds of up to 35 miles per hour, making the run from 43rd to Congress in just 10 minutes.

The CTA^® decided to no longer used the express tracks starting with the North-South service revision in 1949, instead using A/B skip-stop express operations. The express tracks remained unused, and when what now is the Green Line was shut down for extensive renovation in 1994, the express tracks were removed and not restored.

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