Intended to be a part of the planned Chicago-Peoria-St. Louis system, the section between the CO&P at Streator and the ITS at Mackinaw Junction was never built, leaving the former line separate from the rest of the ITS.
The CO&P provided regular service to the cities along the Illinois Valley until its failure at the height of the Great Depression.
Two years later the line was extended in both directions, west from Marquette to Princeton and east from Marseilles to Seneca.
The line was lightly built and never carried much freight, depending almost entirely on passenger service.
In 1929 the Streator branch was abandoned and the west end of the mainline was cut back to Depue.