Mike,
As others have advised you are over engineering your base frame work, however if you have the money to invest there is really nothing wrong with doing it, especially if you plan to making this a permanent layout, that will be up for many many years. My bigger multi level layout was up for about 12 years, and I used 2x6's to mount to the wall studding and the outside platform edging, with 2x4 cross members 3' apart, the under pinning was simply new plastic saw horses, which the 2x4's sat on. The deck top was 3/4 finished Ply Wood. On top of the plywood was the old Acoustical Ceiling Tile, covered with indoor/outdoor carpeting, to completely deaden the sound of the FasTrack layout while the trains ran. In this manner I kept the building cost down, and still engineered a very strong multi-level layout. If you let it, building your layout structure can cost you big money, however, it can be done for reasonable money in the manner I just described.
PCRR/Dave
