The spectacle part of US railroading is the freight train operation, these don't pass through Union Station or downtown Chicago, but start from each of the large freight yards on the various radiating routes, and there are a series of connecting lines through the suburbs between them. Aurora, on the BNSF-hosted Metra line westward, is mentioned above, but is a good place to travel to, pass their two main yards, see action, and sit in a bar beside the line, which as well as the suburban trains is probably the busiest freight route.
There's a webpage here that describes it. In typical USA rail article style it envisages travelling there by car (!), but you can get there on Metra.