A "transdorm" is a "transitional dorm" car. It is used as the sleeping space for the on board staff, but it also has some revenue roomettes that can be sold to passengers if needed. However the transitional part is very important.
All access between Superliner cars are on the upper level only. The Conductor has to specify the baggage that is to be off loaded at a station. Thus the transdorm is placed between the rest of the train and the baggage car. The transdorm has one door in one end on the upper level, and one door on the other end at the lower lever - thus it is a "transition" car, since baggage cars are single level cars only. Otherwise, the Conductor would have to wait until the train arrives at a station, get out, walk to the baggage car, climb in and then locate the bags to be offloaded. With the transdorm, (s)he can do this prior to the train's arrival at the station!