Train Table - Top

I am about to build my train table (4 x 8) and I'm not sure of my next step.

Do I lay the track/ballast directly to the wooden surface or can I add a layer of styrofoam (not sure what thickness I would use) and lay the track on this.

The styrofoam or other material would be used so I can add some contour to the topography. Perhaps stream bed, culverts things of that nature. Major landscaping such as hills would be added separately. Any suggestions for a better material are welcome or telling me this si a bad idea are OK to.

Thanks
Steve