Do not... I say DO NOT.. place track on bare plywood. As Virginian noted, the result will ruin the experience -- and you may never even run the trains much as a result. 
And what fun is that!
Some of the best sound-proofing methods I've seen usually involve some type of carpet buffer between the roadbed and plywood material: either carpet itself or carpet under-padding. You KNOW you've achieved the desired results when you only hear the "clickity-clack" of wheels over the rail joints (when you silence the Railsounds, of course). 
David
