Thanks for the tips.
Exterior grade (APA grade "Exterior" not outdoor treated) plywood, primed and painted twice, is extremely stable. Lightly nail it down on the table frame, with slightly oversized drilled holds and and use common "framing" sometimes called "sinker" nails that are slightly smaller in diameter, and you will really reduce the wood expansion problems. I always lightly painted the wooden frame of the layout with primer too.
I had a huge HO layout as a kid, in an unheated and uncooled attic. Sometimes it was so humid that water would drip down the walls if we didn't leave the windows open. Temperatures would often rise to 100+ degrees up there.
I nailed down every inch of track of my HO layout, lots and lots of long curves and straightaways, and did not experience any expansion problems, using the table method described above. My Uncle was an old school carpenter, shot in the leg by a Japanese plane strafing the deck of his ship in Okinawa, and told me what to do. (He was a gunner's mate on the pom- pom gun.)
Glad to hear that people are using screws on the tracks, 50 years after my original layout. That certainly sounds easier!
Regards,
Mannyrock