From the 1850s through the 1960s, most mail going intercity went by train for most of its journey. From the 1880s through the 1960s, almost all intercity mail went by train. Railroads made Benjamin Franklin's dream of dependable mail service at low cost to every city and hamlet throughout the nation a practical reality.
'Mail cars' came in two types: ordinary baggage cars devoted to carrying full loads of sorted mail between major cities, and Railway Post Office cars. The former cars were railroad-owned cars, unstaffed, assigned to mail service, with all access doors (at the side and ends) securely locked by Postal Department employees anytime there was mail inside.
Railway Post Office cars (RPO cars) were genuine post offices on wheels. U.S. Postal Department mail clerks (i.e., federal employees) staffed such cars and sorted mail enroute. For security of the mail, no one other than USPD employees could enter such a car, not even the railroad's crew or staff. (Clerks wore pistols to ensure that rule.) The USPD operated the cars on scheduled passenger trains all over the nation.
RPO cars picked up first-class letter pouches dispatched by thousands of small-town post offices 'on the fly.' At the train station platform, a postal worker put the canvas pouch up on a special trackside pole. As the RPO car approached, a clerk aboard swung outward a special hook in the doorway of the RPO that snared the pouch. At the same time, an RPO clerk threw out onto the platform a letter pouch with mail destined for that town. At towns and cities having scheduled stops for passenger trains, such dramatics weren't needed; the larger volumes of mail involved were handled during station stops. When an RPO car was stopped, one could mail a letter by dropping it into the mail slot found on either side of any RPO.
RPO cars and RPO staff were each assigned to particular routes. RPO clerks became quite familiar with all the postal destinations along their route and sorted all mail received accordingly. A letter posted from a small town commonly arrived in the post office of another town on the route on the same day. Mail bound for far-off places went into particular pouches that would be sent onto correct intersecting routes at larger post offices. The mail distribution system followed the railroads.
Pre-printed business envelopes used to carry a line at the upper left, "Return to Sender After 5 Days," above the return address. Trains carried mail coast-to-coast in a maximum of four days. With one day for routing to the 'dead letter' stack, that made five days after the date of the letter's cancellation.