I have used 1" thick exterior foam board as a track road base since 1999. It is cheap, readily available, easy to cut into strips or curves and holds up, in heat and wet conditions, (since 1999 in my case). It can be painted or not, since it will be covered with ballast. I am using it on my new elevated layout on bench work, but will paint it this time for additional protection, as the soil/ballast depth will be kept to a minimum (1"-1-½") to keep weight down on the bench work. Protecting from sun damage in the SW, is more important the moisture however, but this technique is also true for moisture. Works for me!