In Canada and the United States, a railway post office, commonly abbreviated as RPO, was a railroad car that was normally operated in passenger service and used specifically for staff to sort mail en route, in order to speed delivery. The RPO was staffed by highly trained Railway Mail Service postal clerks, and was off-limits to the passengers on the train.

The mail or post is a system for physically transporting postcards, letters, and parcels. A postal service can be private or public, though many governments place restrictions on private systems. Since the mid-19th century, national postal systems have generally been established as a government monopoly, with a fee on the article prepaid. Proof of payment is usually in the form of an adhesive postage stamp, but a postage meter is also used for bulk mailing.