This is ONLY available in the UK. Consult the items listed, should be able to find comparable products where you are.
Polycraft Moulding & Casting Resin Kit - Mini (Includes SG2000)
If you want to make 39 coal cars, i think it would be worth the effort, but only make the sides plus bottom and than glue all to gather.As The Shed mentioned, how practical would this method be with our large scale trains ? Both in the actual process and the cost.
Oh yes...some simple copper wire soldered into a mash like concrete re bare maze, this is a must, cous it is to big, clever man Greg!you might need to add stiffening metal to the mold in larger sizes.
Like sand casting metal, i am sure you know a thing or two about this "side hobby"Notice the large amount of flash on the casting when the part is removed? It's at the mold parting lines.
Again box it in just like sand casting, you really don't want to have it "wobbly"I suspect that a more sophisticated clamping system of the 2 halves of the mold is needed.