Mike,
AFAIK you can't do it because of the way tracks and objects are rendered for display. From what I could tell (and Milen explained it to me once) any polygon under track cannot be above 0" if you want the track to show. So, the best I could do was create a Baseboard (the Blue polygon) and adjust the walls so they begin 18" below the baseboard and rise 18" above the baseboard. I left your polygon in, but changed the color to yellow, the vertical position to -18" and the depth to 18" so you could see it.
The 2nd SCARM file is just my Go-To file to see how to do things because it has a lot of elements and was where I first noticed that tracks covered rectangles, etc.
