I have been figuring out a spiral helix that will allow the trains to be seen all the way up the helix to the upper level. It starts out at a 36" radius and ends up at about 26" radius.
I used plywood for the sub-road bed along with foam cut to 1/2" thickness. The next step will be to glue the cork roadbed and then lay the track. It will be four loops when completed and clime about 18" when done. Planning on putting scenery that will have tunnels and bridges to depict the Tehachapi loop area.