The David Rumsey Historical Map Collection has a great assortment of historical maps, including quite a few railroad maps. The interface to zoom in and pan is really well implemented and the maps listed below are wonderful visual references to the importance of railroading in the late 1800s and the turn of the century.
Erie Railroad - 1927
Lackawanna Railroad.
The Road of Anthracite. Through Car Routes
Santa Fe - 1904
Railways of Canada - 1886
Railroad map U.S. - 1890
Denver and Rio Grande Railroad System - 1883
