Hi all
I'm new here, but having read your ideas so far I think you are going to come unstuck with that 6" joining section. Years ago I had a modular layout in 0 gauge tinplate (yes, Hornby) and I joined the sections with small 2-3" sections of track that slotted in vertically onto brass fingers that went up the middle of the tinplate. Worked well, with the loose fit allowing for any slight inaccuracies. Now, so far your moduals are loose in relation to each other? So what if one gets knocked out of line? Shouldn't they be g-clamped together? Then take your loose section and have it (the birch ply with track attached) sit in a recess on each board as they are slid together, apply the clamp or otherwise lock them together - now that they are together. Now, like my joining piece, yours only needs to be a couple of inches long.