The various rapid transit companies originally had their own separate terminals in downtown Chicago. On November 22, 1894, the Union Elevated Railroad was incorporated to construct the Loop elevated line. And on October 3, 1897, the Lake Street trains were the first ones to use the completed Loop line.

From the beginning, there had been talk of unifying Chicago's elevated system. The Loop made unification seem more logical, but the original Loop stations had four separate and isolated platform areas, one area for each company. On July 1, 1911, the Chicago Elevated Railways Collateral Trust was formed as a "voluntary organization" to unify the companies.

Also in 1911, Samuel Insull acquired control of each the four rapid transit companies.

On November 3, 1913, through routed service was introduced between the Northwestern Elevated and the South Side Elevated systems. And at the same time, passengers were permitted to transfer between the different systems without paying a new fare. These changes were made to comply with ordinances passed by the city council.

On January 9, 1924, all elevated companies were consolidated into the Chicago Rapid Transit Company. That company then became a subsidiary the electric utility Commonwealth Edison, also controlled by Insull.

Samuel Insull also controlled three major electric interurban railways entering downtown Chicago, as well as the Gary (Indiana) Railways. Two of the interurban railways used Chicago's elevated lines to enter downtown Chicago. The Chicago Aurora and Elgin Railroad used the Garfield Park line to enter the city, and ceased operations in 1957. And the Chicago North Shore and Milwaukee Railroad entered Chicago via Evanston and Skokie, ceasing operations in 1963.

Insull resigned from control of his companies in 1932. The Chicago Rapid Transit Company remained a subsidiary of Commonwealth Edison until 1938.