From what i can make out from all the various pictures i've seen. Various freights have consisted of various sized plank opens, bogie bolsters, vans of differing types, well wagons, tanks of various sizes, the dreaded 16 t mineral, 13 t high sided steel open etc etc. It's your model, if it's look right, it is right. If all else all fails. Rule 1 applies. It's your model. As an thought, there is a current topic about brake vans, they may be well be out of your time line, but perhaps maybe give you some guidence