Ahhhh, but with NASA's World Wind, you can load up my shapefile of New York State Railroad Routes, and trace every railroad ever graded in New York State.
World Wind: http://worldwindcentral.com/
My shapefile: http://russnelson.com/nyrr/
That same data is also availlable plotted (one line at a time) on Google Maps:
Yes, I know, it's just for New York State. In principle, though, other people could do the same thing for other states. It's a metric buttload of work, but it's only taken me two years of very part-time work and research. My discovery rate for new lines is down to one every other month.