First, I cut out four 70mm x 300mm plates of cardboard, on to which I glued parallel 300mm balsa strips with PVA glue. Once that was dry, I spread a 50/50 mix of PVA-glue-and-water on the cardboard before dredging it in model railroad ballast, and that's that.
Murphy's law of building such model kits is:
The more insignificant and less visible the detail, the more difficult it is to make.
During the game, nobody will pay attention to what's below the train, but I still need to make a way for the train to "glide" on the railroad. I did it with long strips of plastic glued on 3mm-thick cubes. The space between the cubes is, of course, the same as the space between the rails.
A few decorative rivets on the strips, and it's time to test it out!
And for the final touch, the windows: They're simply made of rectangles of metal screen that I superglued in place.