3/8" ply under 1/2" foamboard; is this ok?

[MountainMan]

One factor, however, does remain constant. If you allow you're wood to adapt to the ambient temp/humidity of the house, and then seal it completely, you will drastically reduce the problems of expansion and contraction. By "sealing" I mean all four sides or edges of everything, and the open ends of boards as well. Any bare wood is able to react to humidity and swell slightly, which along with temperature changes leads to expansion.

Also avoid laying track across materials with different reactive coefficients.