If you are as compulsive as I am about getting old maps, head right over to this site. And if you need to get your historical character from Chicago to Butte on the train, some larger libraries carry an Official Guide of the Railways for times past. I had a great time checking out the 1921 version online.
(Thanks to Bill Sornsin, Great Northern Railway Historical Society for the RR timetable tip.)
I’ve also found the Library of Congress useful for maps and emphemera (timetables, advertisments). It’s sort of hit or miss, though.