I had often thought about inexpensive or 'filler' vehicles for our trains. Had the idea of injection-molded shell of generic vehicles, but with highly detailed relief features like model kits, and snap in wheel/axels.
I remember YEARS ago, P&G made a laundry detergent called BIZ that had a promo of Ford vehicles made by Funmate that were attached to the detergent. Cars came with a launcher, were plastic and detailed, and looked to be O scale. Those could make great filler cars, as I have seen them go for around $5 at time on the bay.