Kevin, if it's not too late, definitely don't use #2 pine 1 x 12s. Take it from a guy who made his living as cabinetmaker for about 20 years. Aside from seasonal movement, which is across the width of the board, they will warp and twist. The wood at the big box stores is not dried for interior work- it is only air dried to about 10-12%. Cabinet grade wood should be kiln dried to 6-8%. This means as the wood acclimates to a house that is heated or air-conditioned, it will move unpredictably. I would splurge and buy 3/4" birch ply. It will be dimensionally stable. They can rip it for you on the panel saw at the store into 4 pieces about 12" x 8' long. If you have a friend with a biscuit joiner, that would be a simple way to join them end for end.
As for the corners, the brackets will work where the curve starts if you put battens on the wall from there into the corner on both sides. Putting a longer bracket in the corner isn't really practical and will look worse than battens ( ledger strips) IMO. You need to secure the wood to the brackets and battens, and then you can float the roadbed and track.
I don't see why you would need safety rails, since these are all inside curves and the trains would derail toward the wall.
