DoubleDAZ posted: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.
Hello Dave,
The actual inside dimension on the "4-ft" sections is 4'-1/4" (48-1/4") . The outside dimension is 4'-5/8" but remember the angle is .1875" (3/16") thick per leg / flange so it will wind up being 4'-1/4" inside. I don't know how well my drawing shows the dimensions but that center dimension should read 4'-5/8" and again that is outside.
The frame construction will be in two halves, consisting of an upper section and a lower section and joined together with the vertical columns of 2 x 1 x 3/16 rectangle tube. The "left half" will be welded up as a singular unit and the same for the "right half". The two halves ( units ) will be bolted together at several locations using grade-8 hardware. When the two units are bolted together they will also share 2 of the 4 holes per swivel caster. So they are also held together via the caster plate. I can do a blow-up drawing to better show this if anyone would like to see this detail.
I re-considered the suggestion by Dallas to have 4 each casters per unit to aide in moving each individual unit if the assembly was ever moved and decided to leave it as drawn. The reason being that the plywood deck is not joined in the very center. One sheet overlaps from one unit to the other so if I ever need to disassemble the two units the track and plywood would need to be removed. These frames will not be that heavy ( each unit ) so I do not need the casters to move them. In fact I will not have the caster installed to be able to stand the unit up ( back to top ) to be able to get them into the house. The distance between the bottom frame and the top frame, less the casters, will be 31-1/2" which will clear my standard 36" ( actual opening including the trim is 35" ) doors.
Yes you are correct about the inset on the bottom frame on the Left Unit, it is to clear my fire place on that wall. Not to worry, I don't use the fire place so no roaring fires while the train is on the track 
I think that the 3/8 Luan plywood will be OK, support wise, in the open spaces for a distributed load. Just to be sure I will do a load calculation based on the opening of 3'-10" by 3'-3" and find where the critical loading will be. Thanks for pointing that out, I can easily add a center longitudinal frame and reduce the 3'-10" by half.
I really appreciate the extra "eyes" and experience reviewing my design, thanks again for all the help.
