Ill be using tubular O gauge track on 2" foam with cork road bed under the track and rubber track ties under the track. No plywood base at all.. Everything track and all will be glued to the foam base.
It will very quiet.
Dave
My Gargraves flex track does not have any screw holes - I guess that means no screws are needed. 
I have been "testing" foam road bed and not found it to be all that great at sound deadening (over 1/2 inch plywood). I picked up a db app for my phone and found the sound level ranges from 63 (normal conversation) to 72 db (heavy traffic). Because of the scale 10 db does make a difference. I am using Gargraves and Ross track screwed directly into plywood except where I was able to try the foam road bed. Funny I find my bridge sections with 1x4 pine are the quietest sections in my layout. I'll have to do more fiddling with the meter to confirm, but I think the nosiest sections are the ones with the largest continuous sheets of plywood underneath. One Ross turnout is incredibly noisy (72db) even adding different running sounds as the engine crosses. (I've have to try a homasote section with the Gragraves. But with my first layout with Lionel O27 tubular, it made no difference - incredibly noisy)
I am also finding there is some track flex where foam is installed under turnouts, when the heavier steamers roll over them resulting in the lead truck occasionally derailing.
One screw per Fastrack track piece, always at the same end so the adjacent piece is secured by that screw as well. If possible, at track switches I use a screw on the three adjacent track pieces to allow the switch to float.
