Basically, yes. The Railway Post Office car was a rolling Post Office, where the mail was sorted en route. The mail clerk was a federal employee, not a railroad company employee. Mail clerks had to know what the transfer points were for mail destined beyond their run, so they could get it into the correct pouch, and put it off at the correct stop, and they had to be able to sort mail quickly and accurately, so that they could exchange the outgoing mail pouch for the incoming one without carrying something by its station, or putting something off early.
The 'exchange' or transfer of pouches was probably always a stationary exchange on the D&RGW narrow gauge, but of course on the standard gauge, the exchange frequently took place on the fly - the clerk standing in the car door, ready to kick the outgoing pouch out and manipulate the catcher arm to pick the incoming pouch off the mail crane.
Because RPO cars were built to Post Office specs, the D&RGW RPOs had the hardware for catcher arms, though I don't think I have ever seen a photo of one in use on the narrow gauge.
Charlie Mutschler
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