So, I am taking away from all this that the extruded foam boards can be laid on benchwork. I think I would feel safer using a 1/4" plywood on the benchwork and a 2" exrtruded foam over that, because I can see myself leaniing on the layout and breaking the foam. Does this seem sensible? I had thought of just using the thickest foam I could get and maybe bonding 2 sheets if necessary, but putting thin plywood down first seems more prudent. I am thinking of building this in three 2.5' by 4' sections so it can be moved when we sell the hiouse.
pmcgurin;
There's no harm in putting 1/4" plywood under 2" foam. If nothing else, it will give you a surface to attach switch machines, & wiring, to the bottom of your layout.
However, depending on the framework under the layout, you won't need the plywood, or need to worry about breaking the foam. With the common grid frame of 1x3 stringers every 16"-18" you could lay the foam directly on top of the frame, with no plywood, and be able to climb on top of the foam & crawl across your layout without breaking the foam.
It is possible to slightly dent the surface of the foam by leaning an elbow on it really hard. But the "damage" is superficial at most, and having plywood under the foam wouldn't prevent this anyway.
Building your layout in sections is a very sensible idea. Not only does it make it easier to move into a new home, but it also lets you work on the "bottom stuff" switch machines & wiring, without crawling under the layout.

Any traditionally "under-the-table" job is ten times easier if you can take a section to your workbench, or table, flip it upside down, sit in a chair, and work in comfort. Try it. You'll like it.
If you build your layout in sections, make the frame in sections too. Glue 1 x 3 planks, or better yet, 1 x 3 & 1x 2 L'-girders, (see photos) around the sides of each section. The frames can be bolted together.
Wiring should pass through terminal strips, or plugs, at each joint between sections. I don't recommend the commonly suggested trick of drilling large holes in each frame member and then threading all your wiring through these holes. You may need to disconnect the sections, and a wire "umbilical cord" between them might not be easy to separate. Instead, use cable clamps, cup hooks, tie wraps, or any of the other wire holders that can be undone without sawing into your layout, or cutting the wiring harness. The attached files have more information on benchwork.
Traction Fan 🙂