Marbletown

I can't quite decide if it means he's hiding from a railroad bull (security officer) in a graveyard OR if he was killed by the bull and either doesn't realize it ("I can hear them a- hollerin' We got a man down here") or does is has accepted his fate ("A bone yard is the best").

I think he was killed accidentally by the bull and is subtly accepting his death by pretending he's just hiding for a while out in the graveyard.

It seems to be about vagrants travelling the railroads and sleeping rough among the graves in America, maybe in the early 20th century.

Marbletown is slang for a graveyard, it was (and still is) common for homeless people to sleep rough among the graves, sometimes pushing tombstones off to find a dry place to sleep. Hence the opening line, roll out here mister if you need a little rest - to roll out would signify laying out some kind of bed to sleep on among the tombstones and hoping it doesn't rain.

A "bull" is a name given to guards employed by the railroads to seek out those who would otherwise seek a free ride on a train and throw them off. On finding an illegal traveller on the train you can imagine the bull shouting out, "we got a man down here".

Cannonball is a term given to a non-stop train, because it doesn't stop for anyone, much like a literal cannonball wouldn't, so our imagined protagonist waits his time and rests the night in the graveyard until morning comes when he can seek out an open box car on a train and take his chances once more.