Oakfan,
If you use the 3/4" plywood and put the padding under the track you'll have a quiet layout. Scenery on the ply - paint and glue and scenic material works great - always has.
Using foam as a deck or on top of the deck doesn't really have any advantage. Sure, it is light weight and doesn't require power tools. It brings a different set of attachment limitations and such.
it is good for carving rivers and sanding for terrain shaping.
yes, some are making it work as decking. The old rule applies - it is your layout - do want you want to do.
I, too, confirmed that Home Depot no longer stocks or ships Soundboard440 (homosote) to CA. here is their product info.
Model # 206136
Internet #202090212
Store SKU #123024
Store SO SKU #532811
No big loss in my opinion.
