Homasote/Homabed, cork, foam. There are others, but these are probably the most common.
Foam, as in Woodland Scenics foam roadbed:
soft, sound absorbing, glues down, and is fairly easy to handle, cut, and work with. Track is nailed, and the nails can be removed after the ballast has set permanently. Given the flexibility of foam, I would not personally recommend the removal of the track nails, I don't believe that the foam is strong enough to hold the track and ballast by itself.
Cork:
Most of the same attributes of Foam, but is a little harder to cut, and is strong enough to allow the removal of track nails after setting the ballast. I prefer it to foam (personal preference).
Homasote/Homabed:
Homabed is precut and shaped, and while a touch more expensive than Homasote board, is probobly worth the price, as Homasote requires power tools to cut and shape. Homabed seems to be more tightly packed/denser, than Homasote board. The real benefit here, as opposed to cork, and foam, is the ability of Homabed/Homasote to hold rail spikes. For those who would hand lay track on wood ties, the "Homa---" products are the only way to go (wood on wood tends to be just a little noisier)
For "snap track" (preformed, sectional), and for Flextrack, Cork and Foam are both suitable.
My only caveat, Don't lay cork and foam roadbed directly on polystyrene foam. I'm from the "old school", that believes that a plywood subroadbed on risers, will outlast polystyrene foam subroadbed, and you'll have far less problems with track alignment changes over time.
Pete
