This cargo trailer is both a railcar and a trailer for a truck. The rail wheelsets are removable, and the road wheels are then available to be towed down the road. I built this many years ago and it’s been on numerous BayLTC train layouts, but I never put pictures online until now.
I first learned about these interesting hybrid vehicles on the PBS television show Tracks Ahead. They are a real thing made by Wabash National. Each trailer has permanently-attached road wheels, and can have railroad wheelsets attached for use in a train. The trailers are hitched to each other, with the wheelset mounted on the rear of the trailer, and a special wheelset is attached to the front one.
By the way, when I posted my Gondola car, I said that was my first train car MOC. Technically I guess the RoadRailers are, but the Gondola is more of a “train car” in the traditional sense, whereas the RoadRailers are more road trailers that can be carried as a part of a train in my mind.