Matt -
After lots of issues with heat and paint - I've opted to go with ol' fashioned steel. The Texas heat caused most plastics to sag - sometimes just a tiny bit - but sag nonetheless. And, no matter what I tried, the whack of a weed-eater would peel the paint off of PVC posts in short order.
I'm learning the ins and outs of powder coating for the posts. The other support sections are primed and painted with a high-temp product - and so far (knock on steel) no rust and no sag. The powder coated posts show some surface markings from the weed-eater - but I was basically trying to chew the post down with it. 
I should note that the draw back to the steel is its inherent conductivity. I'm leaning heavily towards battery operated locos as it eliminates any issues with connectivity between track sections. My biggest fear is that a youngin would get shocked if power was run through the track and they touched it.
BTW - kudos to Norm and his Rail Bender!!! It's built beautifully and it works like a champ - right out to the very ends of the flex track. By permanently bending the track - I no longer have to screw it down - which helps to ward off any problems from expand/contract. I'm also working on a retention spring that keeps the track ends together through the expansion and contraction cycles.
After almost two years - I'm in too deep to quit! LOL!
Good to know about the Texas heat. Looking at relocating to Houston myself despite the humidity. Plan to run some tests out here in San Bernardino which has been hotter and more humid than Houston believe it or not.
The earth grounding of the track support will help TMCC operation. There is indeed potential shock hazard from someone touching the track while there's a voltage and also touching earth ground. You could reduce the risk by using a thin wood decking on top of the metal support structure, which would reduce the risk of someone accidentally touching the framework while touching the track. The other option would be to somehow insulate the track support deck from the posts thus preventing the support deck from becoming an earth ground.
Refresh my memory. Which rail bender is Norm's?
