Much of the magazine business was US Mail....
For example, Kable Printing, in Mt. Morris, IL, had a branch post office
right in the printing plant. CB&Q "jeeps" (original and converted 50' AAR
autobox-based Troop Kitchen Cars) were purchased largely (I believe) for
this type of service and were loaded in the plant, sealed by the Post
Office, and shipped nation-wide. Playboy Magazine and the Spiegel Catalogs
were two of the products of this plant.
They were handled in a local mixed freight from Mt. Morris and were taken
down to Aurora where they were put directly into mail trains.
I disremember the proper terminology for this type of mail shipment (bulk
mail, second class??) but at least some portion of the magazine and catalog
traffic was moved as mail.and not Railway Express.
PRR B60, PRR X29, NYC 70' baggage (the cars that appeared in the last two
Mainline Modeler issues), UP horse baggage cars, the "Q" ex-Troop Kitchen
Cars, and the REA/NYC/NH/EL ex-Troop Pullman Cars were all very common in
this service and could be found in just about everybody's secondary
passenger trains (and later in hot freight trains as passenger service came
off). Other roads had cars in this service as well (SP, MP, GM&O, B&O,
etc..) but those listed are my candidates for most ubiquitous.
Charlie Vlk