In 1936 the Nickel Plate Road started to convert old box cars into what they called "riders". These side door, no cupola cars were cut in behind the locomotive on local freights. The head brakeman, extra brakeman (Swingman on the NKP main, Listman on the Clover Leaf District) and conductor rode this car, leaving just the flagman on the caboose. These cars had a desk, stove and bench. They were used to haul company mail and packages, as well as Railway Express parcels. Since there were not enough riders to go around, some locals had a caboose assigned to this duty. My question is, "What other railroads used a vehicle like the rider car, where, and in what capacity?"
This sounds like services usually provided by a mixed train. A mixed train with out passengers, in this case.
Some railroads used cabooses equipped with side doors for this service. Rock Island comes to mind: I've actually been inside a couple of these (converted from small wooden boxcars).
I think generally the practice was to equip these cabooses with extra seats and operate them as a "caboose mixed". Passengers could then be accommodated without needing an actual passenger car.
When I worked on railroads in the caboose era, company mail and small amounts of company material (small parts for locomotives, for example) were generally carried on the caboose.
Les