Windhund this is the perfect time to be looking into this. Lots of us have a wealth of info and experience on the topic.
Open benchwork with plywood base for the sub-roadbed is quieter than a full table surface. I did a full table surface anyway, for the increased flexibility and to more quickly have a playable area.
I used Homasote (pressed recycled cardboard) over 1/2" plywood on flat-mounted 2x4 framework; the flat-mounted 2x4's gave me a little better under-layout clearance, which I needed due to a lower base height (my kids were very young, and I wanted it low enough they could easily play with the outer 2 feet). For further isolation, all legs were on their own frame, topped with carpeted cradles to insulate sound transfer to the floor.
For roadbed and attachment, I tried samples of Vinylbed, but decided I couldn't afford it. I went with cork roadbed, which was easier to work with, though probably not as effective. I glued the cork roadbed - the less the better - stapling into the homasote for a week or two to let the white glue cure, then removing the staples. The flex track I bent to shape, trimmed, screwed down, and ballasted with rubber ballast in a semi-flexible white glue mix. Once the track and ballast was settled in (about 2 weeks) I pulled all the screws except at the track joints; after a year I pulled those too. Later I switched to Ross Custom for curved track, and found I could use a lighter ballast glue mix for more float and better sound insulation.
BTW my first attempt I used brown insulation board from the local big box. This was mostly cellulite but had fiberglass in it too. It was a terrible mistake and took weeks to clean up from, rendering the room unlivable til all the fiberglass has been cleaned out.
Foam board ends up being not as great for sound deadening as I expected. I used pink insulation board over 1x4 framework on my Dad's HO layout and was not happy with the result. I guess its stiffness is too great, and its low weight makes it useless for mass-based sound absorption.
Next time I'll probably do the same 2x4 free-standing base, but will use open-benchwork sub roadbed as much as possible, to cut up the table top and reduce its drum effect. I'll also try adding carpet between the plywood and the 2x4's. I might use a much less dense 1/4" sheet foam to make my own roadbed. I'll look for a more flexible glue for the the rubber ballast. For track I'll use less flex and more solid; I like Ross Custom a lot for its wood ties, though I wish they had solid rails like Atlas (and I wish Atlas had solid ties!).
