That is a super piece of work, many congratulations.
I hadn't heard of the DWWR or DSER using passenger vans until comparatively recently. Like Mayner, I find it surprising, though not impossible if they were built with this in mind. The T & D, LLSR and CDRJC all used passenger brakes as goods brakes, and although they were narrow gauge and therefore the weight of the trains WAY lighter, the gradients were vicious.
Also, in places like South Africa, with gradients almost impossible to believe, such practices were common.