SteveFromPA - I agree it isn't a customized piece someone threw together - it's an exact match for a Marx prototype that sold at auction a couple of years ago. Yours would suggest this version actually made it into production.
Underside of Marx prototype engine
The prototype shell is a KCS diesel...and speaking of KCS diesels - this is one of my favorite sets
It's not an actual "as issued" Marx set. I bought a boxed partial set several years ago which was missing two cars. I decided I didn't care to recreate the set since I already had examples of the original cars so I got a KCS B unit and a C&O gondola and put this one together.