The new center section is ready for the sub-roadbed. I decided to use the cookie-cutter method. I cut a piece of 1/4" plywood to fit the general track area. My conclusion was that the solid spline sub-roadbed made out of Masonite board is definite overkill for N-scale.
My new design has one leg (the one in the back) go down and the other leg go up, so that the two can cross each other on the left-hand side of the layout. Spline roadbed doesn't go up and down easily. Maybe over long distances, but I needed to gain a 2-inch separation in about 4 feet. If I used one-inch thick spline roadbed here I would have had to go up at least 3 inches.
I could now focus on the vertical support boards that will hold the sub-roadbed up. I made sure the first section, where there will be a turnout, is perfectly horizontal and matches the existing main line roadbed from the right-hand side.
I am going to be using cork for the roadbed, so I installed a block into the existing spline roadbed such that the top of the cork will match the underside of the existing ties.
I could then glue the vertical boards in their correct location and at the desired decreasing or increasing height.
The sub-roadbed could then be installed with screws. I didn't tighten them down too much so as to avoid undulating sub-roadbed.
I decided to try a similar approach for the benchwork on the left-hand side. Individual boards of 1/4-inch plywood were cut and placed on the cabinets. Small glue blocks held them upright. These were just loose, because I didn't want to attach anything to the cabinets.
The scenery base/sub-roadbed will be thing that will hold the vertical supports in position. See the official track plan for what this side of the layout will look like. This photo shows the lower-level base.
I have now installed the upper level too.
I decided that the best way to get a flowing connection between the cookie-cutter roadbed in the new center section and the plywood sheets of the left-hand side of the layout was to use the Masonite spline roadbed approach again. The big advantage of spline roadbed is that you get the natural easements built-in. In this photo I placed an 8-foot long, one-inch wide strip of Masonite connecting the two areas to determine how long it needs to be. A vertical block of wood sticking up in the back corner is to prevent the strip from going outside the back section of the layout.
I connected the spline Masonite strip to the outside edge of the cookie-cutter roadbed of the center section using glue. Here the glue is drying.
On the other side, where the spline connects to the horizontal surface of the left-hand side of the layout, I cut a slot in the plywood, installed a supporting block, and glued the Masonite spline in place. This guarantees a smooth transition from one surface to another.
I could then cut some more vertical blocks that will act as the support for the spline roadbed.
I then cut some small plywood blocks and glued them to the outside Masonite strip, and glued the center strip to them.
This, too, was connected on the other end via a groove.
Another collection of blocks and the inside strip of Masonite completed the open-frame spline sub-roadbed, providing a smooth transition from the center section of the layout to the left-hand side.
Here is a higher-up view of the new corner.
I then used a rasp to make sure all the spacer blocks were even with the tops of the Masonite strips.