Sweet tug and barges
Ignore the Numptys
I have heard of people using granules of cork, suitably darkened, for coal. I love the loads in the barges - I always admire the Thames barge builders who take the trouble to include the "payload" and often make the tarpaulin, hatch covers removable to show a load of Medway sand, newly fired Bricks and a wheelbarrow. (or whatever)
andrew