Bob, I can't imagine how much your modules weigh! 
I've always used 1/8" masonite for spline roadbed. For single track, I do 5 layers. However, layers 2 and 4 are just spacers... 1 inch spacers on a 6-8" interval really cuts down on the material used.
For the double track mainlines on Modutrak, we just run the outside spines and run spacers perpendicular. This saves a lot of weight and is plenty strong... the spline roadbed is the spine and strength of our modules.
We cap everything instead of trying to keep the top of all the strips level. A flush cutting router takes the place of any careful measuring when gluing down the cap.
There a bunch of photos in this article: http://www.model160.com/n-scale-magazine-features/franksville-wisconsin-project-part-2-laying-spline-roadbed/
Skibbe, Actually, my modules are easily manageable by my son and me. In a pinch, I can move each one and set them up by myself. The pine L-girder benchwork is very light but stiff and dimensionally stable since I use only premium, knot free pine and every joint is glued and screwed.
Yes, the Masonite spline assemblies are definitely not the lightest way of doing subroadbed, but as you know, they are extremely stable, which is important to me because of the code 40 trackage I have a propensity to include for sidings and branchlines.
I use spacers between the double-tracked mainlines, except where there's a center siding between them. However to keep the tops straight, I just have the splined assemblies planed, which lightens them up a bit and only costs five bucks at my local mill. Truth is, I only do that because it's convenient and makes it so I don't have to breathe Masonite dust if I were to run my sander over them. Since I build them upside down, the tops are pretty even anyway, but I'm a little anal about having a smooth surface to lay roadbed and track on.
However, your system of using spacers for splines 2 and 4 is a great idea (which never crossed my mind before) and I'll have to take a look at that on the 6' Wilhemina Pass module and the 8' Devils Slide module since there'll be a lot of cliffs and rockwork on both of them and weight savings will be much appreciated...especially on the longer than normal eight footer.
As for caps on top of strips with perpendicular bracing between them, it seems to me that would work okay as a monocoque structural element, but it would be noisy and work best if the track is straight. However for long curves running fairly close to the fascia of corner modules, with bridges intersecting it, I have my doubts that it would work well as a structural element. In utilizing a "box" both as a modular structural element and as subroadbed for curved mainlines, it seems to me that it sort of defeats one of the advantages of using Masonite splines, which is to efficiently use the entire sheet. You'd get the same sort of waste that cutting curved subroadbed out of plywood gives you, except Masonite isn't as costly as plywood.
I'll probably stick with planing the spline assemblies and using internal L-girders and gussets in place of a monocoque box.
When I designed the "F-Units" modular concept, which utilizes a "standard" module size of 3' wide (at the common ends) by 6' long, saving weight was not one of my priorities. The main priority was to take advantage of N-scale's scenery to track ratio in modules that pushed the portability envelope. The standard benchwork is light enough, but also very robust with stiff, braced and adjustable integral folding legs. There are absolutely no "plywood plains" on any of the modules, either as a scenery base or as a base layer under 2" Styrofoam, since a plywood top is not needed as a structural element. Towns and factories are set on 2" Styrofoam platforms which allow for topographical carving and are plenty sturdy to support structures.
It's always great to see how the ModuTrak guys do things and it always shows very innovative thinking. I can see where your module construction techniques works great for the midwest scenery that you guys model, but I don't think it would work as well for the F-Units concept which is full of western scenery and includes a lot of canyons, deep gorges and bridges which we have on nearly half of our modules.
