Let me offer a different suggestion/observation after seeing your photo:
1) Finish adding floor tiles/carpeting (think I see some black tiles) before you complete the layout. Soundproofing under the table makes a big difference.
2) Finish the ceiling and overhead lighting also. Basically I'm suggesting you finish your train room before doing more on your layout. It will be so much easier to complete room construction without worrying about damage to your layout.
Yes, I made the mistake of completing my own layout before the room was completed. Fortunately, I had an excellent carpenter/drywall expert to bail me out!
3) No matter whether you use screws, glue, foam, or plywood, be prepared for your relative noise level to increase after ballasting. The rock-solid nature of a ballasted track ensures excellent audio transmission from the track ties to the plywood/foam/cork base. (I once tested rubber ballast but it was too difficult to lay down properly.)
