Railway Post Office (RPO) postmarks were applied to:
1) Letters deposited through the RPO car's letter slot or handed to the Railway Postal Clerk if standing in the doorway;
2) Letters deposited into a street letter collection box mounted on the depot platform that was marked "train mail" or similar endorsement;
3) A misdirected letter was endorsed "MISSENT" and the RPO receiving that letter postmarked the letter on the front, then redirected the letter for proper dispatch; and,
4) Special Delivery letters were back-stamped with the RPO postmark if a clerk on-board sorted that letter.
Letters mailed in a city or town received that Post Office's postmark, not a RPO postmark. Mail with un-canceled stamps were not postmarked in a RPO car; that obliteration to prevent re-use was the destination post office's responsibility.
Mail moved by rail fell into three classifications.
Closed-pouch mail was sorted and postmarked at a post office and shipped as freight. It was not processed by the RPO.
Initial-terminal mail delivered to the RPO at the beginning point of the line was sorted and postmarked en route by the RPO. Processed mail was bagged and off-loaded at the appropriate stations along the line or passed on at the end of the run for forwarding.
Between-terminals mail was picked up at stations after the train began its run. This mail was sorted, postmarked, and if appropriate, off-loaded while the train was en route. At stations where the train did not stop, mail pouches were hooked at speed from small cranes. On some trains, passengers could mail letters or postcards by dropping them through a special slot in the mail car.