Another option much like Martyn has done is to cut up an insertion (ply, plastic coated foam board etc) to where you want the coal, then line the wagon or tender with Clingfilm. Carefully put the insert into the wagon or tender space. Then a dose of full strength PVA onto the insert, now load the coal onto it, dust helps fill the gaps. Next up a gentle spray from a cosmetic spray bottle (dead cheep from chemists) with a drop of washing up liquid. Finally whilst still wet soak the lot with 50/50 watered down PVA again a drop of washing up liquid and allow to dry. Overnight in an airing cupboard or perhaps a day or so in a cool room. Once dry the Clingfilm can be removed from load and wagon/tender if you got it right no glue will have seaped into the wagon/tender.
I have used this method in G, 0, HO, 00 scales and confirm the success of it.