I just noticed that you sent me a message last September! I apologize for failing to respond sooner.
I developed my track plan as an N-scale version of John Allen's famous HO Gorre & Daphetid RR. I was not aware of the Hakuhodo Branch.
I wanted my railroad to be lightweight, which is why I used extruded polystyrene (XPS) foam with 1x4 and 1x3 benchwork. I have no regrets. XPS is very easy to work with and has proven to be sturdy and stable. I used various thicknesses of XPS, from 1/4 inch for retaining walls and abutments, to 3/4-inch for the baseboard and "cookie cutter" sub-roadbed, to 2-inch for the risers.
Once the layout is finished and scenicked, there is no way to tell if it is made with traditional hardshell techniques or with foam until you need to move the layout -- then you know it utilizes a lightweight construction technique.
My response is probably too late to be of any use to you. Whatever you decided, I hope it is working out for you.