The Chicago and South Side Rapid Transit Railroad Company was incorporated on January 4, 1888, and secured a franchise from the City of Chicago on March 26 of that year to construct an elevated railroad between Van Buren Street and 39th Street (Pershing Road). The franchise required the company to build along a right of way immediately adjacent and parallel to one of the alleys from Van Buren Street to 37th Street, rapidly earning the line the nickname of the "alley L". On April 2, 1892 the city authorized the extension of the line as far south as 71st street, and a further extension along 63rd Street was passed on April 7, 1893, the total cost of construction was estimated at $6,750,000.
A 6-car train carrying 300 guests made the inaugural run along first section of the line—running between a station at 39th Street and the Congress Terminal downtown—on May 27, 1892, and the line opened to the public ten days later. Initially the 3.6-mile (5.8 km) journey from 39th street to downtown took 14 minutes and cost 5¢. The line was gradually extended over the following months, with the route reaching Jackson Park on May 12, 1893 to provide service to the World's Columbian Exposition. Rolling stock on the line included 46 Forney-type (0-4-4) steam locomotives that were built at the Baldwin Locomotive Works in Philadelphia, and 180 46-foot (14.02 m)-long wooden passenger cars. The first 20 locomotives were delivered coupled into a single train in April 1892.
The South Side Elevated railroad provided 24-hour service, a major advantage when compared to Chicago's cable railroads which required daily overnight shutdown for cable maintenance. After midnight, two trains ran on the line, providing service every 20 minutes; in contrast, rush-hour service required 18 trains to maintain a schedule with a 3-minute headway.
When the World's Columbian Exposition closed, lack of development along the southern portion of the route led to plummeting passenger numbers. The Chicago and South Side Rapid Transit Railroad Company went into receivership in 1895 and was sold under foreclosure on September 16, 1896 for $4,100,100. The South Side Elevated Railroad was formed to take over the route in 1897. Service was extended into the newly built Union Loop on October 18, 1897 connecting the South Side Elevated Railroad with the Lake Street Elevated Railroad, the Metropolitan West Side Elevated Railroad, and (after 1900) the Northwestern Elevated Railroad. These other companies used third rail electrification to power their trains so the South Side Elevated Railroad enlisted Frank Julian Sprague to convert its rolling stock to electrical power. Sprague used his previously untested system of multiple-unit train control (MU) whereby multiple self-powered cars could be linked together and controlled by a single person, making the South Side Elevated Railroad the first in the world to use MU operation.
As ridership increased, the South Side Elevated Railroad constructed additional branches. A 3-mile (4.8 km) branch to Englewood opened in stages between 1905 and 1907 (with the short 0.9-mile (1.4 km) Normal Park branch opening in 1907), and branches to Kenwood and the Union Stock Yards—both running on tracks owned by the Chicago Junction Railroad—opened in 1907 and 1908 respectively.