+1 on 1x4s being sufficient to support a layout. Over two decades of module building, I have purchased carefully selected 8 ft 1x3s and 1x4s from the big box stores and stacked them in the corner of the shop. There are times when I don't buy any from some the poor stock on hand. These are air dried at least a year before I use them so twist and warp of cut pieces is minimal, but I usually discard one or two out of a dozen. It's wood, and new growth at that. It wants to change shape, sometimes immediately after you cut it to length. If you are purchasing a lot of lumber at once, you can't discriminate that much and may have a higher reject percentage. Ripping 3/4" birch ply to framing dimensions sounds competitive on cost and the wood is stable, so that is the way to go IMHO.
A table with 2x2 legs (braced) will support a LOT of weight. 3/8" carriage bolts and a T-nut allow height adjustment. A 3/4" rubber tip will fit over the end of the carriage bolt.
I found that elevated track sections (4" to 7" wide) cut out of 3/4" birch for seasonal layouts have proven stable even in crappy summer storage conditions. I can't say the same for 3/4" fir or pine ply.
