FWIW, I started with a 6" elevation, but I had to increase it to 7" to accommodate the tallest cars. Right now, the tallest car I have access to is the MTH autorack, it stands 5 1/8" above the rail-head. With allowance for the main level track, roadbed, and also the elevated section plywood and supports, I needed 7" from the main table level to the top of the elevated level surface. My calculations are 1" for the elevated plywood and supports, and 7/16" for the main level roadbed and track. This allows 5 1/2" of clearance, which should be sufficient for anything I'll be running.
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