Another option to help with the sound is to lay 1" thick foam board from one of the box stores on top of the plywood/homasote. Cut it all the way out to the edges of the layout. You can use 1" x ??? trim boards to cover the edge and the 2" x 4"'s, and give a finished look. Put it down with some construction caulking and a few heavy books. You can paint the foam with some earth colored latex and sprinkle the wet paint with green and yellow ground turf and you'll have instant layout. In my experience, the Fastrack will not move very much, if at all, on this surface and doesn't need to be glued/nailed/caulked/screwed down. Once the track is down you can easily scrape away any areas you want smooth for roads and paint/lay them in (or lay them out and mask off beforehand). Seams are generally not visible, but if you want to be on the safe side, I've used 1" wide wallboard tape and flexible vinyl spackling compound before painting.
