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.

The world's first official carriage of mail by rail was by the United Kingdom's General Post Office in November 1830, using adapted railway carriages on the Liverpool and Manchester Railway. Sorting of mail en route first occurred in the United Kingdom with the introduction of the Travelling Post Office in 1838 on the Grand Junction Railway following the introduction of the Railways (Conveyance of Mails) Act 1838.

In the United States, some references suggest that the first shipment of mail carried on a train (sorted at the endpoints and merely carried in a bag on the train with other baggage) occurred in 1831 on the South Carolina Rail Road. Other sources state that the first official contract to regularly carry mail on a train was made with the Baltimore and Ohio Railroad in either 1834 or 1835. The United States Congress officially designated all railroads as official postal routes on July 7, 1838. Similar services were introduced on Canadian railroads in 1859.

...
Wikipedia