The point about warping is important, I have been battling with warped timber this afternoon, timber bought "New" 30 odd years ago which has distorted quite noticeably and enough to create unwanted 1 in 100 gradients, Today 3 hours work lowered the summit by 1/4" and allowed a 9F to pull an additional 3 bogie bolsters up the grade, (or a total of 14 Mk 1 coaches).
Somewhat fortunately the bit in question has trackbeds built on risers so so apart from the summit where the 5" trackbeds join a 30" wide baseboard I was able to adjust it by changing the thickness of the packing etc.
My "Answer" to warping is to buy good dry second hand timber, I get mine from scrapped builders vans in a local scrap yard. Planed and sanded it is as good as new and won't twist like a dogs hind leg after a couple of years.
MDF sounds like a very good choice for trackbeds, the problem I had is the trackbeds are double track width climbing at 1 in 50 around a 3ft radius curve and keeping the track level means quite a significant lateral twist has to be applied and the fibre board used fought me constantly as it didn't like the idea of not staying flat. I used Riser supports every 8 inches or less around the curves.
L girders or other riser based baseboard systems make realistic scenery much easier than trying to do something based on a flat monolithic slab,