Dan,
If you weren't going to be picky, you wouldn't have crafted a response. Anyway, you are indeed correct, as that is an RPO. The Sumpter Valley did have a beautiful little baggage car as well, but the 3 and 4 were used heavily on the passenger trains. Number 3 sports UNITED STATES RAILWAY POST OFFICE between the doors, and BAGGAGE AND EXPRESS on the coach end. Sadly, apart from the locomotive, we would have been able to replicate this passenger train until about 1971, when one of these duck-billed RPO's was being used as a cabin. An overzealous ranger ended that idea though.
Thanks, Taylor