Here is a schematic diagram, front and profile, of the inside of the hoppers. It will allow us to cut pieces of extruded polystyrene foam (XPS), the top of which will then be shaped.
Of course, the precision of the dimensions is illusory when XPS is cut with a cutter. The trick is to print these diagrams on paper and use them as a guide to position the cutter blade.
Here is what I get at different stages of “sculpture”. From left to right, a piece cut but whose top is not shaped yet. In the middle, the part which comes to cap the ballast, and whose shape is simpler. Right, in the wagon, part shaped and sanded.