will screws in foam board hold?

I personally have not tried it but I have heard of others trying it with no luck screwing directly into the foam. If you plan on ballasting the track, use longer screws to reach into the wood. then remove screws after the ballast has dried, The glue and ballast will hold the track in place on the foam.

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. 

The screws WILL hold in pink or blue foam. That was how I secured my FasTrack to my layout. The screws do make the track much louder, even without going into the wood. To reduce that noise, I'd suggest using rubber O rings between the screw and the track/roadbed.

May I ask how long the screws are? Good tip on the O rings, I didn't know they made them that small. Do you use countersink screws and are you able to put the O ring in the countersink hole?

you must run your trains with the sounds systems off.

I am using MTH realtrax.  I can use a thicker plywood, say 1/2 inch, so how long of a screw do I need to get past the track road bed that is built into the track?  I will run it through the foam to the plywood but make it short enough to not go all the way through the playwood.

If you glue your ballast down, you can pull the screws out. For this application I would just glue the track down. I would use Elmer's wood glue, or a tube type acrylic adhesive that comes in a caulk tube.

I thought of screwing the track instead of gluing so I could lay the track after painting and putting grass so none gets on the track itself and then I could move the track if need be or remove it when we move to protect it. 

I used 1.5" or 2" drywall screws, and my foam was 2" thick. For 1/2" foam, 3/4 or 1" screws would work with FasTrack or Realtrax. The roadbed is at least 1/2 tall, so the screws never got close to the wood table top.

good to know, in addition, if my screws do go through the board, I could glue foam board underneath and thus prevent children from hurting themselves when underneath

1/2" roadbed plus 1/2" foam equals 1", so you need at least a 1 1/4" screw with the 1/4" plywood and 1 3/8" should get you in without going through if you use 1/2" plywood. Personally I would go with the 1/2" plywood.

The bigger issue though is going to be noise. I had my RealTrax on plywood temporarily and I was amazed at how loud it got when I moved it to my 1" foam bed, and that is without screws. I've since put a drop cloth under it for a temporary Christmas layout and the noise level dropped a lot. It appears that RealTrax is great on carpet, but foam acts as a resonator. My suggestion is to run noise tests before you glue/screw anything. My LHS has their layout on indoor/outdoor carpet and it is quiet. I'm rethinking my use of both RealTrax and foam. My layout is going to be in my open-plan living room, kitchen and family room, at least for a couple of years, so noise is important to me.

Screws themselves won't hold, but if you plan to ballast the track, then the track stays in place do to the amount of glue in the ballast.

 

If you are wanting to not ballast track, but are wanting something temporary, then screwing the track down may not even be worth it.

 

regardless, all you'l do is put holes in your scenery base.