Penistone Railway Works. Fortunately filling up a wagon is easy.
Firstly, I'm assuming that you will want to be able to easily remove the load, and that for weight and cost reasons you won’t actually want to completely fill the wagon. The solution is to use cardboard to build a base to which we stick just the top of the mound that would fill the wagon.
Building wagon loads in this way is a messy process that involves cling film and watered down PVA glue, but the result is quite convincing. For this example I used coal lumps from Woodland Scenics. It's fairly straightforward to measure a wagon and cut the cardboard to size, but if you are wanting to add a load to a Penistone Railway Works wagon then I've provided free downloadable templates and instructions; for example here is how to build a load for an 8 plank wagon.
The reason I referred to a penn'orth of coal in the title of this post, is that I buried a penny in the middle of the mound. Recently minted pennies have a steel core and hence stick to magnets. This means that I can use a suitably strong magnet to help lift the load out of the wagon and the penny also adds a little bit of bulk to the centre of the mound to ensure that it isn't all at the same level.