Mike, this may be a neophyte question, but who makes that B&O wagontop in HO?
A bit late answering this question, but ...
Mike answered who made the N-scale car he has. I second that they're nice resin kits, the whole Fine N-Scale line is actually. The decals leave a bit to be desired on some cars, but the kits themselves are nice.
As far as HO versions, F&G (Funaro and Camerlengo)
http://www.fandckits.com/ makes rather nice ones in HO. They even make what I think of as the matching 'wagon top' B&O hopper, both of which are shown on their main page. I've seen these kits and handled them, they're rather nice kits also.