I collect really old RR photos, mostly postcards. The laws are complicated and have several different criteria. As I recall, if something is over 70 years old and no copyright on the image (and copyright not renewed,) they are considered public domain. During the 1950s a lot of serious railfans (i.e. foamers) used the Rollieflex. Both Eric Treacy and David Plowden had one. It's almost certain that your negs came from a railfan and not a pro photographer. As to how the negs got out on the market, there are two ways that are most likely. First, the foamers of the time used to trade negs all the time, and later Kodachrome slides. Second, they could have come from an estate sale after the guy died and the kids didn't want a bunch of old photos of trains. That's the more likely scenario. As for your making prints, I say go for it. You've got the negs. Here's a link to U.S. copyright laws. It looks to me that if the copyright was not renewed after 28 years on a photo made before 1978, it expired. I'd say the odds that a foamer had his images copyrighted are about zero.
Kent in SD