I used some foamboard adhesive I bought from Lowes (Same container as caulk). It’s expensive though at about 6 bucks a container and was barely enough to glue the bottom board to the plywood. It seems to work ok unless you pull up too much on the board then it easily pulls off.
Does anyone else recommend something else that is better? Also what do you use to glue boards to each other when you build layers?
Two words: “Latex Caulk”
The cheap stuff is good, as long as it doesn’t attack foam. I got mine from Meijer’s (like WalMart) for a mere .99 cents a tube. Works great for foam to wood, roadbed to foam, and track to roadbed.
I have had good luck with latex caulk, both (blue, pink and white) foam to plywood and foam to foam. However, I’m using cookie-cut foam over cookie-cut plywood, not fastening an entire sheet at one time.
The key to success, for me, has been applying pressure to hold everything together while the caulk sets up. I have used clamps (with plywood between foam and clamp to spread the pressure) and various weights (old phone books, cordless tools and their spare batteries, chunks of angle iron stacked together, full 2-liter bottles of soda, even a 16 inch length of 75# rail.) If the weight has the potential to leave dimples or ridges in the foam I put pieces of scrap foam between - the scrap takes the damage and the main surface stays pristine.
I also use caulk to fasten cardstock (track templates) to the foam, and flex track to the cardstock. Foam to plywood I use white caulk, while above-the-foam caulkwork is done with grey (ballast colored) caulk.
Chuck (modeling Central Japan in September, 1964)
[#ditto] Latex caulk.
Om my website [in my sig] I show how to use white glue properly so it does not pool etc.
I have used white glue for this for years and never had a problem. However there IS a technique to it. I never have used the other types of adhesives myself. I know they work but white glue is a LOT cheaper.
Anyhow, pop over and have a look. I am sure it can save you a lot of money.
I hope this helps you out. 
Latex caulk works well, although SILICONE caulk actually works better for foam to wood and foam to foam bonds. I use pure latex caulk to glue down track. I generally buy latex caulk with silicone by the case as a compromise between the two.
I have tried a glue that was intended for glueing foam (i use the blue). It worked but only barely. It’s easy to peel it of, so it seems to be best suited for temporary gluing [:(].
After that I have tried the carpenters white glue and it sticks very good to foam. The white glue is much slower on foam than wood though. The reason is that the white glue is water based and the water can not evaporate through the foam.