I have decided to go with cork between the track and plywood, not so much for sound, but because I want the track raised a bit with respect to the base. I bought a big roll of it from Amazon, and plan to cut it into 4 inch strips, and 1 inch strips (4 laid together) to use in the curves.
I have about 16 feet of 24 inch deep benchwork complete at this point, and I am going to start laying a little bit of track to see how it goes. Will report back.