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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.
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. The Railway Mail Service enforced various standardized designs on RPOs. A number of railway companies maintained nominally unprofitable passenger routes, having found that their financial losses from moving people were more than offset by transporting the mail on such passenger routes.