I use 3/16 or 5/16 thick Upson Board. This is a pressed paper product that is denser and harder than homosote and not so prone ot absorbing moisture. It comes in 4x8 ft sheets like plywood and is bascially white on both sides.
On my layout I cut it to strips a little wider than the ties length and just put in under the track. I cut slots to be able to bend it on curves.
This is probably not as quiet as using a carpet type product.
