I just came across a very interesting reference resource, thanks to a link on a French modeling forum. I don't know if it's been posted here before. This site has several digitized books by a Walter Gilman Berg, which feature plans and info about railroad structures:
http://archive.org/search.php?query=walter%20gilman%20berg (http://archive.org/search.php?query=walter%20gilman%20berg)
One of the books is titled, "Buildings and Structures of American Railroads", and is a 1908 reference book for railroad managers, superintendents, etc.
There is also a similar book on American railway bridges and structures, and another on railway shop systems.
They can all be viewed online, downloaded to an ebook, or downloaded as a pdf.