Lou, you are quite correct; they are hard to find, out of the box.
The solution is to make them yourself. What you bash may not follow what real railroads used exactly, but great enjoyment can be had just trying.
One car that was used quite a bit during the steam to head end power transition, was the "Steam Generator car.
Considering that a lot of the model Diesel locomotives available were not modeled with any sign of a steam generator on board (Such as the Aristo, Alco f, and FB units), a steam generator car is needed, for heat.
I made up several “Steam Gennies” using two Lionel plug door box cars. I used Bachmann passenger trucks to represent higher speed passenger trucks. Too bad I don’t do photography as a hobby. I will try to get a photo taken, if one of the guys comes by.
Another car that can be bashed is an “Express Reefer”. Again, all it takes is a common "Bachmann, LGB, or USTrains, wood reefer, mounted on Bachmann, or LGB passenger trucks, and painted with your road’s paint scheme, and lettering in the letterboard.
Quite often, you can put an Aristo heavyweight baggage car in the consist of your stainless equipped passenger train, or even a combine. The real railroads mixed up their equipment more often than not…try it, you might like it.
Fred Mills