The plaster idea is a good one, and I have used the technique back in my model railroading days. Two things I would add, though.
1. Molding plaster is easier to work with than plaster of paris, which tends to set up too fast for meticulous work like this. Molding plaster will give you about a half hour to work after it has been mixed.
2. If you are going to use plaster be sure to seal it completely with some sort of dull finish after you color the pavement to make sure plaster dust doesn't over time creep into the workings of your engines and cause undue wear.
Bernie