I think what I will do is put down a 1/2 inch sheet of plywood and then put a 1 1/2 or 2 inch thick sheet of the heavy duty blue styrene. Then paint it a light brown with some cheap paint. Then I will use some liquid nails and lay the track directly onto the blue foam board. With 2 inches I know I will have enough room to cut out a small stream which I think will run into a lake which I will only show a very small portion of in one corner of the layout.
I guess I am not being completely clear, I apologize.
I use white styrofoam because it is the most readily available in So Cal.
I have areas of track on inlaid 1/4" cheap plywood, areas of just cheap plywood and areas where the track is laid directly on the foam, with no significant issues with any of them.
I do not believe that the trains will ever weigh enough to embed the ties in the foam.
Since you are using blue foam, which is significantly more dense and durable then the white expanded foam, you should lay the track directly on the foam. I use Titebond II for the foam to track bonding. More working time then liquid nails. I would consider using the 1/4" ply underlayment if you use 1 1/2" or 2" foam.
Once again I apologize for the confusion,
Now we know you are actually human and make a mistake now and then. I still regard you as very knowledgeable and a person whom I would listen to for advice.
When I get home I am going to sort out my Large Scale stuff and sell a bunch of it. I will probably keep my Indy engine and a Porter but will sell my Annies, Connies and a Shay plus a bunch of box cars, gondolas, tanks and stock cars leaving me with about 10 freight cars and three Bachmann passenger cars. That should keep me busy enough outdoors in the summer.
I have been watching a bunch of Youtube videos on making scenery and I am getting excited about that. It should be an interesting winter next year.