The 53' containers have a sliding chassis that extends from 48' to 53'. The 28' UPS containers have dedicated 28' chassis. Frequently, railroads own the chassis, and keep many on hand for incoming trailers. This helps to prevent Denver ending up with hundreds of chassis and Chicago having too few, for example. There are several types of intermodal rail cars.
Spine cars are just that, a metal beam with platforms on the ends of the cars.
Spine cars can usually carry both containers and trailers, but only single stack containers. Most of these cars have been adapted to carry trailers inside the wells with a collapsible fifth wheel on each car end. Well cars are manufactured by several companies, most notably, Thrall Car and Gunderson. Older 89' flat cars are also used in intermodal service. The 89' flats can accommodate two 45' trailers or a 48' and 40', or smaller combinations.