The Central Pacific Railroad Photographic Museum maintains a page with an absolutely mind-boggling amount of information. Sifting through some of their online offerings is well worth the time. There are loads of great photos, links to primary source documents, timelines, and answers to just about any question you could think to ask about the transcontinental line.

The Library of Congress has a very cool—and extensive— railroad maps collection, that helps viewers see the West as the surveyors sketched it and as those who mapped it first.

The Union Pacific's corporate web page has a link to history and photos that hosts information from the UP museum.