R is for Railway Post

illustration of a rail engine with Railway Post Office
Railway Post Office Cinderella Stamp

A Railway Post Office {RPO} was a special mail car on a train. From the 1860s until the 1950s, most intercity mail in the U.S. was postmarked and sorted by hand en route in these cars. While a train was still in motion, RPO clerks used a long hook to pick up mailbags hung on special track-side poles. The last true RPO made its final trip between Washington D.C. and New York in 1977.

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Envelope addressed to Red Cross Founder Clara Barton

The Boston Streetcar RPO picked up this piece of mail, and a clerk postmarked it a 4 a.m. on July 24, 1911. The Boston Circuit RPO ran only at night, collecting and sorting mail from suburban post offices. United States, 1911.