Railway post office

A railway post office, commonly abbreviated as RPO, was a railroad car that was normally operated in passenger service as a means to sort and deliver mail. The RPO was staffed by employees of the postal service, and was off-limits to the passengers on a train.

In the U.S. such a car was also known as a mail car and was equipped and staffed to handle most back-end postal processing functions. An interesting feature some of these cars had was a hook that could be used to snatch a sack of outgoing mail hanging on a track-side pole as the train passed it. Mail cars as such fell into disuse once USPS determined that it was more advantageous to separate the functions of mail transport and mail processing.