I found a good online source of old topo maps many from about 1900.
[
www.lib.utexas.edu]
Click on the state you are interested in and then look for the Historical Topographic Maps tab.
A few I found interesting:
Rico District Colorado showing the Rio Grand Southern and switchbacks to mines.
Santa Fe Mew Mexico showing the "Santa Fe Southern" Chili Line
Cranberry North Carolina showing the ET&WNC before the extension to Boone NC
Are there other online sources of old topo maps that are helpful?
Paul