Excellent tree dt!
About wagon loads. If you make a block to fit loosely,then glue a washer on top in the middle before forming a mound, painting black and adding the coal. A magnet can be used to lift the loads out giving the option of running empties!
I use plywood blocks and roofing tar which comes in one of those mastic gun type tubes. That way no painting is needed and you can just dip the block into a tub of coal and you're done.
The roofing tar might melt your P.I.R though.
Edited;.................................................... with my specs on.