As built, the RDC3 consisted of a 17 foot Railway Post Office, a 17 foot express room, and a 40 foot, 49-seat passenger section. Some RDC3's were built without RPO apartments, but used an identical carbody to those so equipped. Of the five RDC3's originally owned by the New Haven, #'s 126 and 129 migrated to Pennsylvania DOT ownership, where they were converted to all-coach layout. At first they were assigned to Pittsburgh PAT service, later they sent to SEPTA service in Philadelphia. Still later they were sent to Morrison-Knudsen, converted almost back to the original configuration, but without the RPO apartment, and sent to Alaska.

Another New Haven RDC3, the 130, having been converted to a self-propelled track inspection car, found itself in the employ of the C&NW. The carbody modifications were numerous. On the right side, the RPO door and window were retained, but the baggage door was filled in using matching flutes below the belt rail, replacing the opposite side's RPO window with stainless sheet above the belt. The second and third passenger windows were removed and and relocated to the left side in the former location of the baggage door. That displaced door frame was shifted forward to the position of the left side RPO door, which was discarded. On the passenger end, all cab windows, both end and side, were deepened to within a foot of the floor to provide the observation room. Although the letterboard duplicated the graphics of the original 1950 RDCs, the sides of the ex-NH 130 featured two sizes of C&NW logos, and the car's front end was painted with green and yellow diagonal stripes.