This axle configuration is common in the Pacific northwest of the U.S. On the real truck the single wheel pusher axle raises and lowers. On the model the pusher is modeled in the down position. The trailer has been modified so that the tires ride close to the body like the prototype. The trailer has been retrofitted with a shallow 24" kingpin.
Other features include
7" diameter stainless steel stacks, polished to a chrome-like finish