So... you're going to have to cut a 2 foot hole in the table anyways.
If it were me, I'd mount it flush to the bottom of the plywood. Trace the tracks in the top, rout 1/4" deep grooves in the foam top and sink the tracks into the foam top. This will land your ground level roughly even with the tops of the ties. You can ballast in the grooves, which would leave you with an appropriate look for yard trackage. Similarly, rout out around the TT pit so the walls are level, and use a stanley sureform or some low number sand paper to contour the edges of what you contoured back up to the base level and do some scenery.
