my business lives and dies with the USPS, I did some digging for you:
if mail was worked in the car while en route, the car had to be lettered: United States Mail, Railway Post Office. Some RPO's had a mail slot near of next to a door for convenient platform mailing.
some railroads had dedicated 'mail storage' cars, and were so lettered. during times of high mail volume, express, boxcars and reefers were pressed into service as sealed mail storage cars. the modified PRR X29 express boxcar is one example.
mail sorted in an RPO had a 'RMS' or similar postmark obliterator cancellation, noting the route or district of cancellation.
