Imagine an industry so huge that it had an office in every town of any size; that was a hub of techological innovation; that was indispensable to travel and commerce everywhere; that employed and maintained records nationwide -- some kind of hybrid of the internet and the automobile and computer industries.
That was the railroads a century ago, the grimy metallic heart of the nation. One reason we don't consult their records often is a failure of imagination -- these days trains are at best a sideshow in our lives. Another reason is that the records are scattered and in many cases have been destroyed, especially employment records. But it's still worth looking, as a recent discussion on the APG email list reminded me. (Also a hat tip to Paula Stuart Warren's blog post about the Minnesota Historical Society's good work.)
Best overviews (not necessarily best as to current record locations, all URLs as of 29 July 2012):
Wendy L. Elliott, CG(sm), "Railroad Records for Genealogical Research," National Genealogical Society Quarterly 75(4): 271-77 (available free on line to NGS members). As an overview, more thorough and authoritative than anything else linked to in this post.
David A. Pfeiffer, "Riding the Rails Up Paper Mountain: Researching Railroad Records in the National Archives," Prologue vol. 29, no. 1 (Spring 1997)
David Pfeiffer, Records Relating to North American Railroads, Reference Information Paper 91 (Washington DC: NARA, 2001)
Current information on Railroad Retirement Board records (for long-term employees post-1936):
RRB's two-year-old statement
NARA Record Group 184
The pension claims series within that record group
Union records such as this premier African-American union can cut across company lines:
Brotherhood of Sleeping Car Porters, Chicago Division, 1925-1969, at the Chicago History Museum
The more information you can gather ahead of time on your railroad research target person, the better. It's often key to figure out which line he worked for. The sites below vary greatly and are not a substitute for Cyndi's List on railroad records. or the historical society list at RailroadData.com. The libraries and archives below often have great finding aids. The railroad historical societies often cater to modelers (the re-enactors of the train world) more than to historical research.
A good place to start is the railroad-related holdings of Chicago's Newberry Library.
Baltimore & Ohio
Friends of BN genealogy referrals
Newberry Library holdings on this line are in process, but you can check out their blog "Everywhere West" and a photo collection. Actually I really like that blog, especially as a way to wade into the records gradually!
Chicago & Northwestern Historical Society
Great Northern records at the Minnesota Historical Society.
Illinois Central at the Newberry Library
Illinois Central Historical Society
Monon workers index
Northern Pacific at the Minnesota Historical Society
Pennsylvania Railroad at Temple University
Harold Henderson, "Remembering Rail Records -- How To Get On Track," Midwestern Microhistory: A Genealogy Blog, posted 1 August 2012 (http://midwesternmicrohistory.blogspot.com : accessed [access date]). [Please feel free to link to the specific post if you prefer.]
a combo of GM/Toyota and Microsoft back in the day