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

[Tyson Rayles]

My layout ( N scale) is 6 foot by 28 inches and is on an open grid of 24 x 28 inches. On top of that I have 1/8 inch peice of paneling ( it was left over from another project) and a minumum (It's a mountain layout) of 1 inch of foam where the trains run, less foam in one spot where a creek flows off the layout. I have central heat but no A/C so the temp runs between 65 in the winter to low 90's in the summer with humidty in the winter around 40% and as high as 85% in the summer. It's been 5 years and no problems so far.