Hmmm

I don't hear it "Six feet of mud and a shovel muggin'"
Don't make no sense t'all, hardly.

I hear "Six feet 'o mud; I sure been muckin' it." I.e., I sure been in the muck. How you gonna mug a shovel? Besides, suckin' and muckin' rhyme; suckin' and muggin' don't.

Yes, I thought you knowed. Rhymes with road.

"I been doin' some hard travelin'" is also the way I heard it.

I'm trying to think now about the train, because that strikes me as odd too. The passengers are the people, who are blinded by the cinders (esp. if you're a hobo hanging on outside the car instead of passenger sitting inside it).

I been ridin' them fast rattlers, I thought you knowed.
I been ridin' them fast rattlers, way down the road.
I been ridin' them fast rattlers, flyin' cinders, blind passengers;
I been doin' some hard travelin', Lord.

Finally, contrary sod that I am, I don't think a fast rattler is the same as a flat wheeler, if that's what was meant by "as above." A flat wheeler would indeed be a bad trip; a fast rattler is just a train going like a bat so all the couplings and anything else that's loose rattles.

(I'll flag a fast rattler, I'll ride her on down, boys!
They're runnin' the bums outta town. ---Utah Phillips)

Chicken that there Charlie