Well, I've always wanted to map employment density against CTA service, but the data sets are limited. We have a state dataset called Where Workers Work that's by ZIP code, and there's the census transportation package, which is by tract but can be (I hear) hard to work with.
We think transit ridership varies directly with employment, but it would be interesting to see how closely the two trend lines match over a 30-year period.
FWIW, I'm the designer of the CTA map, both the big folded system map and the diagrammatic rail map that CTA uses with a Loop inset. I have a version that I use for other clients where it's all one diagram (no inset) but it's
very tight in the center.
It would be nice to have the chronological record of surface system service changes mentioned above by David, but that's a huge GIS data input exercise rather than a data viz project. Slightly easier would be the rapid transit lines only, as in
this map I did for the
Encyclopedia of Chicago.