Mail hook - AbsoluteAstronomy.com

A mail hook is an installation at a railroad where a mail sack can be hung, to be picked up by a passing train without the train having to stop. See further discussion at Railway Post Office

Railway post office

In the United States a railway post office, commonly abbreviated as RPO, was a railroad car that was normally operated in passenger service as a means to sort mail en route, in order to speed delivery. The RPO was staffed by highly trained Railway Mail Service postal clerks, and was off-limits to...

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