Personally, I'd compare the dimensions to those for standard gauge cars in S scale. I already converted a $1 die cast S scale steel flat car body to On30 by adding a set of On30 trucks. I'm considering converting some of the common American Flyer 630 hoppers over, too. Width is the same, height is the same, the length is just a little odd compared to the Bachmann On30 cars. I think the 40' flat car in S works out to like 34 feet in On30. (I'm too lazy to do the math to figure out the exact length).
So it might be possible to use an S scale steel box car as a starting point for a model of one of these cars in On30.
I don't think it would be too challenging to scratchbuild some of the simpler cars, either, I was thinking of doing a Carson & Colorado combine/caboose from some ancient RMC plans and maybe to save some trouble fitting it to a Bachmann coach frame.
So it might be possible to use an S scale steel box car as a starting point for a model of one of these cars in On30.
I don't think it would be too challenging to scratchbuild some of the simpler cars, either, I was thinking of doing a Carson & Colorado combine/caboose from some ancient RMC plans and maybe to save some trouble fitting it to a Bachmann coach frame.