I started with the benchwork configuration (two 4X8s cut as shown). Then I worked the track plan. I like to use 2" 4X8 sheets for the table top, so I could trim the table top to follow the track. I use 1" foam sheets to layer up the the elevations. I like to use Woodland Scenics risers to get the track grade correct supporting the track, then fill in the spaces with foam sheets cut to fit.
Woodland Scenics risers are based on 8' lengths, or 96 inches (not quite 100 inches). So their 3% grade is actually 3.125% grade.