Brekina and Viking makes hundreds of HO scale autos for railroads. European autos and trucks mostly, but they offer a limited number of US automaker's products too.
It might also be noted that Illinois Central ran helper locomotives in the middle of a train more than a little. I would often see these trains as a boy in my home town on the main lines with a GP9 or GP35 in the middle of a long coal train.
We had an Al's Machine Shop in my home town. Every hot rodder in town took their heads, blocks, and cranks to 'Big Al' for some of the best machining in the Metro East. He could port match and polish with the best of them.