I used OSB (7/16") for my underlayment (uniform strength in all directions) with 1/2" Quiet Brace loosely cemented on top. After considering GarGraves flex, I chose the good ol' tubular O gauge track so that the insulation board could absorb the noise; the 40" sections reduces the frequency of the clickity clack. I attached the track with #4 X 1 1/4"(?) panhead black screws but with a rubber washer next to the tie. Works like a charm. Between running at realistic speeds and the above system, my Hudson and my Northern w/o sound on, run amazingly quiet.
Wally