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I really like the 3/4 ply as roadbed (it's like having 10 scale feet of bedrock underfoot) but you have to watch it like a hawk because it has a mind of its own. In particular, I've been having to check and recheck the grades between risers because this stuff has it's own ideas about how to bend when you're dealing with curves on grades, which is pretty much all the track on this shelf. This has made progress somewhat slow since I really want to nail this for future reliability.
Are you going to cut away the plywood subroadbed, when it is time to install the actual bridge? If so then it might be advisable to install an extra riser on either side of the bridge, about say 6"-7" from the cuts (looks like you already have the 1x4s there to attach them). That should help ensure that the gradient of the subroadbed is preserved when the bridge portion is removed, so you won't have to worry about any additional unwanted flexing.
Awesome progress Gary! It's like watching William Hood's crews giving shape to his grand design (ok, maybe without the blasting powder, but you get the idea )
On the 'highway ramp' section, have you checked the clearances for double-stack cars (with track and roadbed in place)? Also, are you going to have enough width on the subroadbed to support the tunnel portals (BTW let me know if you need a sample casting or two to play with -- I found a few extras I had made from a while back).
Another thought, if you're considering installing any temporary track (to check out grades, train lengths, etc.) I recall seeing the folks at La Mesa installing cardstock strips on the sides of the roadbed, as a safety measure against derailments. Drywall shim strips seem like a quick & easy way to make that, and they're easy enough to tack on
Are you going to cut away the plywood subroadbed, when it is time to install the actual bridge?
Maybe I missed it somewhere in the 10 (!) pages of thread here, but are you including Kern Junction in your plan? It would be a great excuse to split-up staging yards.