Re: Railroad postal workers

Mail clerks in the RPO car took in mailbags from the post offices along the route, sorted it as to destination, either their far terminal and beyond, or other local offices along the way as appropriate.

They also took on mailbags at their starting terminal that contained mail for the post offices along their route. They sorted that along the way as well and put it off at the various station stops.
They had plenty of time to do all this between stops, usually.

They all carried .38 revolvers on their hip when on the car.

You could mail a letter to anywhere on a RPO car at any stop.

Pretty darn efficient when the highways weren't like they are now.
The mail went through regularly and almost never failed.
Early airmail even was "trained" when flying weather stopped it
enroute.