It depends on what's between the plywood and the track. On my layout with green felt, there are 4 or 5 screws on the whole layout. Lightly screwed in place, with no pressure on the track as cited above.
On my modular layout, I use ty-wraps to secure the track. One goes thru a hole drilled in the plywood, a second tywrap cinches the first loosely from underneath. The track can actually move about 1/16". Works perfectly, even transporting the modules.
