A valuable reference for archaeologists, historians, modelers, and anyone else interested in early trains, a compilation of bulletins 1-10 of the Railway and Locomotive Historical Society is available for free online. (At 617 pages it’s a rather large download, so people with slow connections be warned.)
Although published in the 1920s, most of the articles deal with 19th century railroading. The bulletins are weighted toward Eastern U.S. railroads, which is not surprising to anybody who has seen a railroad map of North America. There are, however, a number of articles on western railroads as well, especially on the Central Pacific. There are also a few dealing with lines in the U.K. and Canada.