My un-educated guess would be a Mantua box car to go with the General 4-4-0. Just a guess mind you. Is it plastic or metal? Were the couplers the loop type?
Roundhouse/MDC makes those fox trucks, and a line of old time cars, but I don't know if they ever came as a flat kit. I also don't recognise the couplers, but it must be before the advent of horn hooks.