I used 3/8" anti fatigue matting. Lowes used to sell it, a 30" wide roll sold by the foot.
I cut it into strips with both sides beveled, using my bandsaw.
It made the layout quiet but as soon as I applied ballast it caused the sound to increase.
The matting turned out to be cheaper than cork roadbed. You could even cover the entire surface of the plywood, fasten it down with spray adhesive.