I did some small tests with fastrack using plywood, ridged foam over the plywood, and rubber padding.(the thin web like material used on shelves and in drawers) I found the foam over plywood was actually louder than just bare plywood. The rubber padding was less so but still noisier than I liked. I didn't test carpet or pad so I can't speak to that. When I build a layout I will use atlas track with some sort of pad (foam or rubber) because it makes the atlas track almost silent. Any track with a molded in roadbed is going to be impossible to deaden the sound.
