Next (below) came covering the insulation boards with plaster cloth, using the Woodland Scenics product.
(Below) Testing the look ... so far, so good. The next step, from a suggestion by professional model builder Tom McKenzie (a member of our Thursday night group, the Gandy Dancers) is to cover the entire surface with dry wall mud. That will be followed by paint, the addition of rock outcroppings, and finally trees, hundreds of trees.
I've got a long way to go, but I'm enjoying finally starting on scenery, and since the reason for the P&LE to exist is primarily hauling coal, the coal facility seems to be the logical place to start.