Mail and Express cars
Railroads hauled the U.S. Mail under contract from the Government. Each railroad had cars called a Railway Post Office (RPO) used to sort mail by Postal Workers and Mail Storage Cars for bulk mail in sacks. Most passenger trains had an RPO Car that would pick up a mail bag on the fly by means of a special hook mounted on the side door. This hook can be seen in many of the RPO photographs listed below.
Mail Trains were slower because of the time making a stop to unload and load bags of U.S. Mail and Express including Milk and Cream. Cars were also picked up and dropped off along the route at railroad junctions from other Mail Trains.
Here is an interesting old photograph showing an SP Baggage Car at the Red Bluff, Ca. freight station during the mid 1950s.