This is covered in at least one other thread on the forum. One tried and tested method that works is to use gorilla glue to glue plastic wall anchors into the foam and then screw into the wall anchors. I have been doing this for about 9 months for all kinds of track work and it works. I am using gargraves flex track with wooden ties. I use the track screws that gargraves sells. I drill holes in the ties where I want the screws to go. Then I bend and position the track as desired. Then I poke a hole with a long thin nail into the foam. When I have the holes poked (I often lay several sections at once), i squirt gorilla glue into each hole. Then I push the wall anchor into the glue-filled hole and wipe off the excess glue. Then I lay the track down in place and start the screw into the wall anchor finger tight. Then I wait for the glue to dry--two hours to overnight. Then I screw the screws all the way in using a Black and Decker electric screwdriver being very careful not to overtighten the screws. There is no need for the screws to be tight--not loose either--just enough. If the glue has expanded out of the hole, I cut off the excess with a hobby knife. Done. I haven't done any ballasting yet but I expect that once the ballast is glued down on the track the screws can be removed because at that point, the ballast glue is going the work. But I have not tested that theory yet. The wall anchors I use are Hillman H#370337 8-10 x 7/8".
Good luck.
Don
