Hobo

To quote the hobos' own definitions:

A hobo is an itinerant worker, a tramp is an itinerant non-worker and a bum is a non-itinerant non-worker.

A hobo was a member of a distinctive sub-culture of homeless, travelling workers in the United States during the late nineteenth and early twentieth centuries. They were particularly associated with the railroads, as they had the reputation for hitching free rides from place to place in the baggage cars of trains.

Hobos in fiction:

• James M. Cain: The Postman Always Rings Twice[?]
• John Steinbeck: Of Mice and Men
• Upton Sinclair: The Jungle (the part where Jurgis escapes from Chicago)

etc.