Hi Papa Dave,
My postwar layout has an upper loop and I used Lionel trestles to support the O gauge track at each joint. On top of the trestles are 1X4 boards that give a little more stability to the track. I also needed 1X4s on the trestle bases to increase the height for clearance. I used small screws to fasten the track through the ties into the 1X4s. The 1X4s are fastened to the trestles with small machine screws and finally the trestle bases are screwed to the plywood base. You can see that I used twin trestles at some points to span the track below. This system has worked out to be quite stable.
Here is a link to a video of the layout which might be useful: https://www.youtube.com/watch?...jANYXPBFw&t=103s