A number of private (mostly volunteer) groups exist to help exchange historical information about railroading. Most, but not all, of these clubs deal with defunct railroads, although many include
information about successor lines. Note that many of these organizations are run on a shoestring and thus may have moved or ceased operation since I last wrote to them. If you write to ask for
specific information it is considered good form to either join the club or at least send a small donation - the person answering your letter isn't getting paid to do so.
NOTE: Each year Model Railroader includes an extensive list of railroad historical societies in the February issue.