Foam works well for shelf layouts as long as it's thick enough to support itself between the brackets.

I was on report (sit waiting for work to happen), and had to some time to cogigate.

I can use 3/4" alumimnum angle stock to create a tray for the the foam to sit in. Attach some more angle stock to brackets and the tray sits on those. It would further need some cross bracing angled much like a bridge truss. With the shelf supports 16" apart & the cross braces, there should be plenty of support for the foam. Sort of | \ | / |.

Doug M

 iF YOU USE 2" THICK FOAM YOU REALLY DON'T NEED ANY SUPPORT UNDER THE FOAM.  As long as the shelf brackets are on 16" centers.  The foam is really more rigid than  most people believe.....Mike     

OOps  on the caps :) 

Good idea but some problems. I used it on my railroad and had to liquid nail some luan to the bottom so I had something the switch machines could get screwed into for support. Also, hand throws do not always set correctly to be firm enough to hold the switch points against the stock rail. I've had to make some modifications so good electrical contact is maintained.

Foam does provide for a lightweight and durable foundation, however.

 had to liquid nail some luan to the bottom so I had something the switch machines could get screwed into for support

You could just cut pieces of luan or hardboard where needed for Tortoises

 hand throws do not always set correctly to be firm enough to hold the switch points against the stock rail

Here you could again use Liquid Nails and set plaster around the stand so it can't move.

I will be using foam insulation board almost exclusively on my new layout (except for major grades) since most of where I am modeling is relatively flat.  At first I considered plywood base but the added expense won't be justifiable given the rigidity of the foam.

I I've been using Caboose Industries hand throws on my foam-based layout for quite awhile now.

Three suggestions:
   - use a very sparing amount of white glue to secure the throw to the surface.
   - use cork roadbed and mount the hand throw on an upside down chunk of that.
   - use long stick pins with green and red heads on them to hold the throws down - they serve to secure and act as indicators of the throw position.

   Where you've got to mount the throw on the foam because there isn't any roadbed under the track, excavate a small hole and glue a piece of cork inside it to use as a base for the throw.