Hi Dougie,
How are you? I am surprised that you would want to make a do over for the holidays after the expansion last year. Well, you have touched on project dear to me.
I started this type of project before, first for a 4' wide table, then to be portable modules to sit on LCCA modules. I learned a few things that can definitely help you. First, though, a couple of photos a of the unfished portable pieces that will work well with how you set-up and break-down and store your holiday layout.
Some lessons:
The train going around a mountain is the desired effect - multiple loops around require too much space and do not provide a great enough bang for the work for more turns.
The base is 4' x 4'. I used 1/4" luan for the base. It is not sturdy enough. Home Depot has 4' x 4' pre-cut project sheets. Use plywood of a thickness that's not too heavy. This was the failure point.
I used a cookie cutter style for the sub-roadbed and blocked it at elevation points with beveled tops on the blocks equal to the grade/slope. This worked well.
The mountain was of two countours. One has a slot to fit together like the old Plasticville pine trees. I glued a piece of dowel to top inside and put a removable screw eye to provide a lift point for placing and removing the mountain. The screw is finger tight and is removed after installation. later supports were installed as stiffeners.
The mountain is to have aluminum window screen stapled to the contours and shaped. Then I am going to squirt Great Stuff on the screen and carve it. This stuff gets hard and makes a nice shell, but it is work to carve it. (more on that later if you want). I got this technique from a Dave Frary book. The screen would have followed the roadbed edge at sat on it, with the very bottom undecorated. The outside of the spiral would have foamed and carved to roadbed height.
The other catch is exiting the mountain in the direction that you want to go. You can see that it required me to put some straights in at the bottom and create a tunnel.
This version was a mix of O36 & 048 FasTrack to get the angles correct for the exit.
It rises to ~ 8".
I also made the reverse loops fit on a 4' x 4' so that the North Pole (elevated section) could also be portable.
So, do want to work something like this on to your L-shaped base from last year? here's what it would have looked like in a 4' x 12'.