Something to take into account, songs originating on back porches would kind of stay in one place for a while, disseminate very slow. At least that's what they do on my porch...

On the other hand, trains would crisscross the land pretty fast. So a new train song or a new version of one might thus get disseminated pretty rapidly.

If there were any (legit) passengers on a freight train, they'd ride in the caboose. Brakemen and various other railroad people changing from one train to another and coming and going different directions. And I suppose that's where some of these gritty kinda train songs originated.

If you never have, go to a railroad museum and look at a caboose. I doubt they rode smooth like a luxury coach with air suspension, nor quiet at all; swaying and bouncing magnified by the last third of the train. My dad rode relocated from Omaha to California in the 30's in one of these. He never mentioned that it was amusing. And probably not much to do, so if there was a banjo I reckon it would get used.