What you probably saw was the McKinley Bridge, which was owned by the Illinois Terminal, a onetime "heavy" interurban operation which ran to Springfield, Peoria, Champaign, etc. There was also a local St. Louis-Granite City service. The IT also carried carload freight, both linehaul and as a switching operation in the Alton-Granite City area (where, originally, it used steam power). The bridge was IT's entrance to St. Louis, again for both passenger and freight. Diesel freight operations continued after passenger service died and the electrification was dismantled. The IT was taken over by a consortium of railroads that entered St. Louis, but I believe it later passed to the N&W exclusively.