Hi,
Your first stop should probably be the various historical & technical societies for the railroad that you want to model, many of them have very detailed and expansive data archives. Then expand your search to other websites and don't forget about railroad and of course also model rairload magazines. If you are living in the area of your model's prototype, a trip to the library can also yield great resources like historic maps.
Here are a few random links, none of them specific to the Southern, but just to give you an example of what kind of stuff is out there:
http://www.gn-npjointarchive.org/http://www.alphabetroute.com/http://www.prrths.com/