Bill,
Quick implementation of your rough plan:
As Eric suggested, having the switches to move trains between the inner and outer loop adds alot to operation.
Also as Eric suggested, if you can afford the space, having the outer loop at 072 or 084 would allow you to operate the larger scale locomotives.
If you are indeed limited to a 6 foot width, this is a nice plan. You might want to consider some access hatches in the middle to make reaching the middle more manageable.
-bruce