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. The first-ever sorting of mail en route occurred in the United Kingdom with the introduction of the Travelling Post Office in 1838. In the United States it was introduced on July 28, 1862 using converted baggage cars on the Hannibal and St. Joseph Railroad (which also delivered the first letter to the Pony Express). Purpose built RPO cars entered service on this line a few weeks after the service was initiated. Many American railroads (the Atchison, Topeka and Santa Fe Railway being just one) earned substantial revenues through contracts with the Post Office to carry mail aboard high-speed passenger trains. After 1948, the railway post office network began its decline although it remained the principal intercity mail transportation and distribution function within the Post Office Department. There were 794 RPO lines operating over 161,000 miles of railroad in that year. Only 262 RPO routes were still operating by January 1, 1962. After 113 years of railway post office operation, the last surviving railway post office running on rails between New York and Washington, D.C. was discontinued on June 30, 1977.

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