Yeah that's the key question - which road? There are several sites like
www.gn-npjointarchive.org that are very good for the particular railway in question. Another option is to contact (or even join!) the historical society of the railway you're interested in, and they should be able to help. And then, there's also various railway-specific Yahoo groups that can also be quite useful.
Another thing that can be useful is finding old Sanborn Fire Insurance maps of the city you're interested in, they're pretty good in showing railway trackage and arrangement of buildings. Many universities have them in their (map) libraries for their own state. Many have them scanned and online, too. Some are freely accessible (South Carolina at SCU for example), whereas others you have to be a resident of the state (North Carolina - their public library system has them, but you have to live in NC to get at it... BOO!), and yet others you need to be registered at or have a library card at the university (Michigan - I think it's UMich that has them; also Montana).
I do have a few for Michigan, NC and SC.