I too used carpet strips as ballast on my Super O layout. It looks great, and does indeed quiet the rail noise. I used indoor "Burber" style carpet. I found that visiting local carpet stores, they had all sorts of reminents I could choose from. I selected a greyish, off white. The rough surface of the Burber weave really looks good for this application. As mentioned, it makes life much easier when you want to make a change. It's also reusable!!
Paul