Hi Thomas,
It was a good suggestion from Bob to change the title since the thread has grown. That said, if I understand correctly, the top and bottom frames will each be built in 2 sections, then joined in the middle. Therefore, I think it makes sense to have a set of casters on each end and in the middle to support the joint. I was going to suggest 4 sets spaced every 2 sections, but then I noticed the bottom frame has a cavity on the left side, presumably to clear something along that wall, so that nixed that idea.
I am somewhat concerned though that the frame is over 4' wide and that means a standard 4' wide sheet of plywood decking won't quite cover it. Your previous photos of the frame didn't include a measurement for the width, but your post on 12/13 suggested it would be less that 4'. In fact, looking at all the dimensions, the frame is slightly larger all over than the baseboard we've been working with. This may be by design, but I thought I'd mention it.
One thing I'm not sure of is if there's enough support in each section to keep the 3/8" plywood from warping when you add rolling stock and buildings. Your sections are approximately 3.5x4 & 4x4 and most folks place cross members every 16"-24". I know you plan to add Homasote or something to deaden the sound and I suspect you'll be fine, I just thought I'd mention it in case someone has experience with 3/8" plywood that size.
Other than that I'm curious to see how the plywood on metal framing works out, I don't remember seeing it before, but I doubt it's a first.
