I came across this website that has some incredible shots of our railroading past from early 1900's to the war years of the 40's. Don't think it has been mentioned here so I will post it and highly recommend it.

http://www.shorpy.com/historical-railroad-and-train-photos

Many of these excellent photos are from the Library of Congress. The images that were taken for the U.S. Government do not have a copyright, and can be used (I believe with credit) for free.

They have more on their Flickr page. http://flickr.com/photos/library_of_congress/

This image by Jack Delano is currently my desktop:
Link