South Carolina Railroad took the first stack of mail aboard in 1831 and the Baltimore and Ohio (B&O) began hauling mail in 1831. It wasn’t until 1835 that the B&O, which ran between Washington and Baltimore, established regular railway mail service was established.
In the 1860s Col. George Buchanan Armstrong, assistant postmaster in Chicago, told Congress that time could be saved and mail delivered faster if it was sorted on trains. Congress procrastinated, but William A. Davis, assistant postmaster at St. Joseph was impressed and experimented with service on the Hannibal & S. Joseph (H&SJ.) The first mail car was a converted baggage car filled with an old letter case and a mail-sorting shelf. H&SJ mail service began on July 28, 1862.
In 1864 Col. Armstrong informed the public of how much faster mail delivery could be. Following the stir crated, Postmaster General Montgomery Blair authorized him to experiment with the idea. Armstrong equipped the Chicago & North Western with a remodelled baggage car. The Travelling Post Office, as it was called, was a huge success.
Following World War II there were 1500 Railway Post Office (RPO) routes with 30,000 men working on them. By 1972, that number had decreased dramatically to two trains on one RPO route between New York and Washington.