What's under your track?

I have decided to start building my layout, and I'm sticking with the O27 tubular Lionel track because of cost. My question is, what should I put under it. I've heard of:

Carpet: great sound deadening, but potential to gum up gears.
Cork: great sound deadening, but expensive?
Styrofoam: what a mess to cut
Blue foam sheets: seen in HO and N, but never O
Homasote: what's that?
Nothing (direct to wood substrate): definitely the cheapest way to go.

I've also heard of zip-tying versus screwing track down.

What do you have/recommend?

Where are you planning on building your layout? I am assuming it will be on a sheet of plywood? If that's the case then I would say avoid carpet. Like you said it can get into the gears, and I can't imagine it looks all that great when you apply scenery. Placing something like cork will certainly help sound deadening, but if money is a factor it's certainly not necessary. You can get away with plain ole' O27 track. Get some wood for ties, add a little balast and it'll look great.

I usually opt to screw down. I've never heard of wire ties, but I used screws on my X-mas layout and my mini layout and the track stays where it's supposed to. You're gonna have to drill for zip ties or screws, so you might as well make it screws. They're cheap enough when you buy em' in bulk. Just one word of advice, screw them in by HAND, not by gun. DON'T over tighten. While screwing in, as soon as the metal tie starts to warp, STOP, and back off just a little to keep it straight. If you were to crank it in, it distorts the track leading to derailment problems. Don't ask me how I know

I've never worked with foam before, so I can't comment on that. I've used cork on HO and it certainly helps deaden the sound (plus it gives a nice look) My O / O27 stuff I've always screwed just straight to the wood but keep in mind it'll be LOUD. Hope I've helped somewhat.