rail-britain has it spot-on. Completely filling wagons will make them too heavy. Plus, if you have a derailment...! :roll:
I used shaped pieces of thick balsa wood for my modest (?) rake of 22 HAAs.
1. Cut the balsa slightly smaller than the interior of the wagon, and carve and sand the top to a mound shape.
(I added a small fillet of balsa to the underside; this is a "rocker" which assists in removing the load, should you wish to).
2. Paint the balsa black, and allow to dry.
3. Coat the upper surface with neat PVA.
4. Apply your crushed coal. Allow to dry thoroughly before tipping off the excess. You may need to remove a little overspill from the sides too.
5. Place into wagon. Job done.
Alternatively:
To miss out Steps 2 & 3, paint the balsa with cheap matt black acrylic paint (used neat) & apply your crushed coal directly to the wet paint. Allow to dry.
HTH,
Don