I'm using RealTrax on 1" foam glued directly on 1x3 bench work and it stays in place just fine without any fasteners. If your layout is stationary, screws into the foam might work just to keep the track from drifting, assuming the foam is the high density stuff, but my guess is normal vibration will cause them to work loose over time. My biggest concern would be transition points for any elevated tracks. If you have track plus roadbed, it looks like you'll need pretty thin 1 1/4" - 1 1/2" screws.
In my case, I don't think screws into the foam only will work when my table has to be moved, so I plan on using long screws to hit the benchwork in key places. I already know a 4'x8' oval will hold with just 4 screws on the corners into wood, but I don't know it that will be true if I switch to ScaleTrax with/without roadbed. I will be tackling the problem in late February and I guess I'll find out then.
