Beginning in 1832 train cars were used to carry the mail, and then in 1864 postal officials inaugurated the Railway Mail Service. Instead of simply using train cars to transport mail, clerks on board specially-designed Railway Post Office (RPO) cars sorted mail while moving.
For stations where the train did not stop, mail cranes were erected next to the track for an exchange while the train passed. Before the train appeared, the postmaster suspended the town’s outgoing mail in a tough canvas pouch on the crane. As the mail car passed, a clerk on board maneuvered a hook to grab the pouch and bring it into the car. Any mail for that town was tossed off the train in a similar pouch. This system was known as 'mail on-the-fly'.
Railway Mail Service carried the majority of U.S. mail for many decades. As the postal service decentralized its operations, it concentrated on sorting much of the growing volume of mail while it was being carried on the nation's rail lines. Not only were mail cars added to trains crisscrossing the nation, but special “Fast Mail Trains” comprised of nothing but mail sorting cars helped move massive amounts of mail between some of the nation’s largest cities. The service ended on June 30, 1977.
Nancy A. Pope, National Postal Museum