Abstract: An refined intermodal vehicle for forming a train of highway trailers. The intermodal vehicle has a unitary upper frame assembly provided with leading and trailing load carrying surfaces spaced below ends of a continuous drawbar to which highway trailers may be connected, each of the highway trailers having leading a trailing coupler socket assemblies having vertically spaced apart aligned apertures. In the refined design a coupling pin passes through the continuous drawbar and tightly engages the apertures in the associated coupler socket so that the coupling pin can rock within the drawbar as the associated trailer rocks relative to the upper frame assembly. In addition, the upper frame assembly is mounted on a pair of steerable lower frame assemblies by springs, and an improved steering mechanism is provided. Other features include a centering ramp, a backup coil spring suspension system, a novel stop mechanism. A novel transition vehicle is also provided.