I use regular carpet (Thunderstorm is the color) on my entire layout. It looks like ballast. It is held in place with the screws that secure the K Line Super Snap track. The sound of the locomotives far exceeds the sound created by anything else. The cut edges of the carpet get "sealed" with a light passing of a torch.
All of our club modules are made in the same manner. It is the quietest I have ever heard for a layout. (pun-intended! )