Table Surface

My son and I, for the previous Christmas, made for the grandkids a layout just the way you have been considering.  The foam was painted with green latex paint.  Although the foam is grooved from running autos   the color has stayed okay.

I assume you're considering 1"-2" of foam on a plywood base with added layers for mountains. As long as the kids aren't crawling around on it, I'd think it would hold up quite well. It is bound to get dinged, gouged, etc., depending on how rough the kids get. If it were me, I'd cut the river, add the mountains and then cover the rest with carpet squares from a remnant/discount place. I might even intersperse some masonite areas for the buildings and roads.

Mine is 6' wide, so 1' wider than yours.  But....

 

I decdied to use cabinet grade plywood to ensure the surface was smooth and the top was even.  It was still hard to find non-warped sheets.  In your case, if you can go 8x8, that is 2 sheets.  If you slice 2 feet length-wise off one sheet, you can get an 8x6 and if you want to add 2 feet to make it 10x6, use the piece you just cut off.

 

For the 'surface', and to reduce noise, I used fatigue mats that I purchased from Harbor Freight.  They are black, but once you put stuff on top it is not that noticeable.

 

Very little of anything is nailed or screwed down to the top.  The weight of everything keeps it in place.

 

The substructure are 2x6s.  Legs are from Ikea (I have my table low so the kids can access it) - 29" high.

 

The plywood is screwed into the sub-structure.

 

I started out at 4x6.  It is now 14 x 6.

 

That's my 2 cents.  E-mail me if you want more specifics.  Good luck.

I am putting 2 layers of 1.5" foam on my layout so I can sculpt deep cuts.

Note, foam does not kill noise much as it just goes through foam.

I am using Celotex Soundstop for a top surface. It eats sound like cotton candy.

It also costs less than homasote, at least around here.

If you are adding foam or sound barrier on top, cheap plywood will suffice as the finish will not be seen.