Securing Fastrack to plywood

#4 it is.

Yes, #4 flat head (remember that the holes are countersunk).

Also note that screwing the Fastrack directly to plywood will probably make it so noisy that it'll wake up the neighbors.

Whether you attach directly to plywood or add a foam base or roadbed:

1. No need to screw every piece down - one every 3-4 sections depending on size and location is generally sufficient.

2. I avoid screwing switches down.

3. If the screw head sticks up too far, use a small countersink bit to slightly enlarge the hole so the screw sits down in the hole without having to overtighten the screw.

4. Do not overtighten.

5. See # 4.

I use one screw per Fastrack section. What with the known issues of electrical continuity I don't want that track moving at all. I have learned to be very cautious and to draw screws down until the track is juuuuust snug on the decking, most especially if a screw is applied to a track switch (which happens only when adjacent track sections don't have screw holes).

At the switching-speed trains mostly move on the PER noise just isn't an issue.   I enjoy listening to the wheel-clicks on the rail joints and with locomotive sound on at low volume everything sounds good to my ear.