Many railroads, Santa Fe included, have operated mail trains almost since the beginning of railroads in the U.S.
For many years, the railroads offered the fastest transport of mail between major cities and over long distances.
The name "Fast Mail" meant just what it said.
The mail trains had expedited schedules, and good, reliable locomotives. They also offered passenger service, for an extra surcharge, to passengers who wanted to get to a destination on the fastest schedule, and didn't mind no-frills accommodations.
The first mail cars were wooden cars, then steel heavyweights, and finally the lightweight cars that you are familiar with.
On most railroads, the local passenger trains relied on a large volume of mail and express transport to generate good revenues to offset the expenses of passenger service.
You may not have known, but the mail trains of yesteryear also served as the primary transportation method of transporting the packages and boxes that are transported today by FedEx, UPS, and other delivery services. Back in those days, the REA (Railway Express Agency) was the local pickup and delivery company. REA trucks met each passenger/mail train to exchange packages and boxes to/from local sources. Even before the REA, there were companies all across the US that collected and delivered express packages to /from railroad trains.
So, if your favorite Aunt Edna wanted to ship you a bunch of Christmas presents, she would contact the REA office to come by her house and pick them up, deliver them to a mail/express/passenger train for transport to the railroad depot nearest to where you lived, and then delivery by an REA truck to your home's doorstep.
For many years, the railroads also provided quick express service for small-sized shipments through a special service that was available at many of the larger railroad freight stations. Called Less-Than-Carload, or LCL service, some railroads even had specially designated freight cars assigned to this service, with one example being the New York Central. These LCL cars were either attached to the Fast Mail trains, passenger trains, or if the volumes of shipments were large enough, separate LCL express trains were operated between major cities.
ACLfan