Just some other ideas, as you won't have access to the sides:
[Above design uses Atlas C55 flex, #7 & curved turnouts, and a #3.5 wye just for reference. Hand laying allows for custom & more compact trackwork]
Pinch the center in, but not too much to create impractical curves.
Above design carves in 16", leaving only 20" to reach over.
Raising the rear track(s) even an 1" (2% grade over the 4' between turnouts) creates a scene separate from all the foreground action.
Also makes it easier to reach / clean the back tracks.
The rear track can be the main (especially if using the curved turnouts in the corners), with the two closer to front acting as passing / staging.
Left Back:
Curved turnouts provide a smoother transition to the back, as well as creates longer runaround tracks.
The example on the left maintains a 16"/15"ish radius all the way through the curve.
Right Back:
Using a smaller radius curve for hidden track can free up 3"-4" front or back.
Really depends on what equipment you run and on what radius curve they're happiest.
If you don't like curved turnouts, you can use #7 or larger straight turnouts.
This makes the middle track the main.
I also recommend access holes in the corners so you can reach up from underneath to clean track /grab gremlins that derail cars back there.
You can also have removable structures (held in place by tabs or magnets or ?) to access from the front.
The industrial trackage in the middle is not set in stone: you can adjust to whatever industries you want.
I do like how one of the sidings on the left can act as a tail track for the "yard" on the right, so you can organize cuts without fouling the main.
But all of that depends on what kind of ops & industries you like
More food for thought.
Hope it helps.
