For those who were not aware: Between 1938 and 1941 the entire state of Illinois was photographed in detail from the air, and the resulting photos are available on-line. The result is a fascinating and almost addictive source of information on the many changes that have occurred over the last 70 years. This link sends you to an index by county. The scans are available as jpeg, sid, or tif files. I find the jpegs to be more than adequate; the resolution is limited by the original photos, not the scanning.
Here, for instance, is a small part of a single photo.
It shows Exposition Park, north of Aurora. No changes have been made except for cropping. This entire site was obliterated by construction of the tollway and its Rt. 31 interchange in the 50's, so this is the best information we will ever have on the layout of this popular attraction. And there are four railroad routes running through this scene. Of course, the railroads are our main interest. It's much easier to trace the path of the Chicago Aurora and DeKalb, for instance, from the standpoint of 1940 (although the line was abandoned in 1923) than it is with modern aerial photography, in spite of its much better resolution. As I say, it can be addictive. Play responsibly.
And by the way: 1) My thanks to the Illinois State Geological Survey, Prairie Research Institute, and U of I for making this wonderful resource available!
2) I haven't tried to find if anything similar is available for other states. Further info would be appreciated.