You could. It would depend on the thickness of the top from the metal or magnet. Or you could put one at either end where the load may be thinner.
The load idea is making tarpaulins with cling film first then PVA soaked tissue paper - toilet roll, kitchen roll or more expensively tissue paper for your nose!
A similar method, cover in cling film, then lay the sheets on the load or wagon (or even coaches to go in sidings) and cover with a layer of tissue paper - enough to reach the ground if it is for coaches in sidings - use string for the ropes and leave to dry. Once dry trim as required, giving the shape and idea of tarpaulins. Paint in a dirty green or brown for effect.
Now you wagons can have covered loads, full loads of sand, gravel, ballast, salt or coal (or whatever) or be empty.
Flatbed wagons can have shaped loads underneath when making the tarpaulins (cars, tanks, pipes) and when the the tarpaulin is dried the 'load' can be reused as the shape will be retained. The purpose of the cling film is to protect either the wagons or special loads from the glue.
EDIT on 14/2/2021: After posting the comment I found this video posted a day later:
https://www.youtube.com/watch?v=cil9b_iHR38