Along with several photos of the ceremony marking the completion of the transcontinental railroad in 1869, the Central Pacific Railroad Photographic Museum page includes scans of a few contemporary newspaper chronicles of the event.
The University of Michigan now has online a full version of The Pacific Tourist, a classic 1876 account of the transcontinental journey by rail.
The Virtual Museum of San Francisco has an online text of Theodore Judah's Pacific railroad proposal of 1857.
The Central Pacific Railroad Museum site has a link to one of Asa Whitney's Pacific railroad proposals to Congress, this one from 1848. The proposal is excerpted.
This scan is the text of war department correspondence regarding Gen. William T. Sherman's 1867 order to provide military protection to trains on the UP route.