Really fascinating stuff here, Steve. For me the ultimate dream is a permanent outdoor tank playground and Village, complete with a true 1/16 scale garden railway. Though I know I probably won't live long enough to make it happen with all the projects I've already got on the list, I can't help but look at this and wonder how I would make it functional. The one photo you show of the underside of the carriage in red and yellow makes me think I could make a frame from aluminum and the frame would be all the parts that are red (maybe just start with a sheet of 1/2 inch plate aluminum and mill away all the areas I don't want. That would be pretty easy.) and then 3D print the yellow plastic side parts for attachment and use wood planks just like you want to. I believe European rail cars of the era had the coupler attached to the frame of the car, not to the frame of the truck. I saw instantly what you meant about the truck being modified for the model railroad car.
Anyway, great stuff, and I can't wait to see where you go from here.