South Carolina Railroad took the first stack of mail aboard in 1831 and the B&O began hauling mail in 1832. Regular mail service began in 1835 on the Baltimore & Ohio, which ran between Washington and Baltimore. In the early 1860's 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 & St. Joseph. 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 Colonel Armstrong informed the public about the faster mail delivery using mail cars. Following the stir created, Postmaster General Montgomery Blair authorized him to experiment with the idea. Armstrong used the Chicago & North Western with a remodelled baggage car. The Travelling Post Office, as it was called, was a huge success.

The use of mail cars peaked by the mid-1900's. Following World War II there were 1500 Railway Post Office routes with 30,000 workers. By 1972 that number had decreased dramatically to two trains on one RPO route between New York and Washington.