Thanks Patrick.
I should have mentioned that I'm looking my Atlas O inventory to be used as the track. Would use my other track inventory (Real Trax) but it doesn't come in O-63 diameters.
I think that I would have the upper level loops off centered in relation to the benchwork so that on the New York side the tracks are on the outside edge (against the wall) so that they can be hidden by buildings and a scene block (Tri-Rail doesn't run in NY 
) with the buildings and road on the inside edge of the loop; and on the South Florida side the tracks are running on the inside edge of the loop with the roads and buildings towards the outside edge.
I didn't mention it before, but on one of the 4 sides I would one day like to have mountains with the trains never being seen as they're in tunnels to represent the countryside of Jamaica in the Caribbean where I grew up (as a city kid). It's beyond my current skill level to build mountains and winding one lane roads with homes dotted about, but I would like to get there.
Came across this HO Scale plan called the Heart of Georgia (HOG) that's providing some beginning inspiration for the overall concept with a big hole in the middle:
