What is "railway post office"

In the United States, a railway post office, commonly abbreviated as RPO, was a railroad car that was normally operated in passenger service as a means 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. In the UK, the equivalent term was Travelling Post Office (TPO).

From the middle of the 19th century, many American railroads earned substantial revenues through contracts with the U.S. Post Office Department (USPOD) to carry mail aboard high-speed passenger trains; and the Railway Mail Service enforced various standardized designs on RPOs. In fact, a number of companies maintained passenger routes where the financial losses from moving people were more than offset by transporting the mail.