cool things with wheels
Quite a regular occurrence back in the day actually...with the advent of mass produced cars and the resultant lack of passenger demand, railroads met the smaller traffic needs with these and similar vehicles. Some, like the RDC and the gas-electric rail cars, were built specifically for this service, others were cobbled from road vehicles like Pierce Arrows, Mack trucks and buses.
I'm guessing, depending on location, rail bus's transported rail workers & supplies. May even, pulled rail cars.