When I had the layout in Dewey, I used 3/8 minus decomposed Granite for my roadbed and let the track "float" in it. With the heat expansion of brass track, I found it the easiest to maintain. That was just my choice.
A "cloth" weed barrier was used under the entire layout area, covered with 3 inches of DG, then the track laid on that and more DG added under the track to form the roadbed. Other landscaping was added over the base of DG as well..
We moved from the Country Club in Dewey to a retirement community in PV a year and a half ago so all has been dismantled and sold..