Well, "Come al you rounders if you want to hear" does not mean that the intended audience of the singer/writer was primarily railroad men. If you're used to singing in saloons, among other venues, it seems to me you'd call your regular audience by a term that fits 'em, even if the subject was something else. In other words, even though it's a railroad song, it doesn't have to have been aimed at railroaders by its writer.

So if "rounders" means lay-abouts and drunks, who else do us songsters get to sing to, anyway?

Bob Clayton