No one has mentioned that RPO's are route specific. The U. S. Post Office had pretty stiff requirements for the service. If I recall mail space was paid for in terms of 15', 30', and 60' space in the car, depending on the volume of service. Only busier lines would have a full RPO. Some trains had more than one, but not very many. Others would have a baggage/RPO combine, and this includes most branch lines and short lines. A full RPO would be of no use to me. And I suspect that the MT cars will come at a price that will cause me to cringe, at kitbashing, given my poor fine motor skills.
Frank