Years ago I saw a track plan in I believe an atlas snap track book . It was towards the back as I recall and I am thinking it was a 4x8 with a 4x4 piece on the side. What made this different was , it had a cut out on the plywood you could trace out to make a different elevation with a jig saw. I would sure appreciate some information on it.
I dont know what specific plan you speak of but back in the day Atlas used what they called a “cookie cutter” method to build their plans. The idea was to draw out the plan on plywood and then using a portable jig saw cut where needed. Add some risers on a wood base and there you have it.
I build an N scale layout from an atlas plan doing the same thing.