On the PRR, it was not uncommon for New Haven RPOs to run through to Washington on overnight mail trains, or to see PRR RPOs on the NH. However, that was the exception as most RPO cars remained on line only. Railroads did lease RPO cars for short periods of time, the most notable being the ATSF 60' RPOs, on lease to the New Haven in 1967 -1968.
Bob's great looking pike sized train is using a ATSF RPO, lettered for the ACL. This reflects the fact that most prototype RPO cars are not commercially available. Athearn and Walthers have run the 60' ATSF RPO, and Rivarossi and Walthers have produced versions of the PRR BM70, and Walthers has produced appropriate RPOs for their series of Featured passenger trains, and later released them in all sorts of road names. I'm fortunate that model the PRR, and numerous classes of PRR head end cars are available. Not every modeler is so lucky.
Boris