Trying to figure out the mass transit of Chicago is like trying to untangle a fishing line. Damn near impossible. I went to this wonderful site - Places OnLine Chicago - and while very helpful, still not as great as I'd like. Maybe because I don't really use mass transit (I find it confusing). Still, it has some great stuff on subways and their rail lines. Plus a boatload of facts, mostly courtesy of the Chicago Public Library.
Gotta love those libraries.