If you're going to go with cork I'd recommend the cheap latex caulk. I use it for tons of stuff and it works great......good for sound deadening too!
For my own roadbed, I took the less traveled (or never travled?) path.
When building my current layout I was looking for a cheaper alternative to the foam roadbed I used last time and with none of the drawbacks associated with cork (it dries out and gets hard and brittle as well as acting like a noise amplifier when it gets that way). As I was walking through the local home improvement store I spotted something that might work, and even better, it was out of season so it was on sale!
I use a foam base with my roadbed and track secured by the latex caulk. For roadbed I use rope caulking. It comes in a roll about 15' long, and it's perfect width for HO scale track right out of the box. Since I model N (obviously), I just tear it along the already formed centerline and I get about 30' of roadbed per box with no waste. The advantages are that it never dries out, so it's always soft and pliable (absorbs sound!), it can easily be molded to any curve radius you want, it's easy to cut with just a xacto knife to form turnout pads, crossovers and such, or you can use a wallpaper roller on it to flatten it like a ramp and make trasitions in rail heights or lower passing sidings in just a few seconds. As if that isn't enough, the latex caulk holds well to it, but you can still slip a putty knife in there and take up the rails without damage if you need (Spread the caulk very thin with a putty knife when you lay it down and then lay the tracks and use some canned goods laid lenthwise along the tracks to hold it while it sets up). If you get it in the off season like me you can often find it for a buck or two a box. It comes in gray and brown that I've seen, though other brands may come in other colors.
Here's what I bought...........
http://i83.photobucket.com/albums/j319/pcarrell/Autumns Ridge/Picture016.jpg
It comes in a roll like this........
http://i83.photobucket.com/albums/j319/pcarrell/Autumns Ridge/Picture019.jpg
And I use a wooden wall paper roller (the kind used to do the edges when hanging the paper) to smooth it and create trasitions in rail height easily.
http://i83.photobucket.com/albums/j319/pcarrell/Autumns Ridge/Picture024.jpg
It's flexible, and stays that way for many years, so it's quiet, and it's very easy to work with.
http://i83.photobucket.com/albums/j319/pcarrell/Autumns Ridge/Picture023.jpg
I can lay dozens of feet of it in just minutes, and since it's just a little tacky, it stays where I put it with no further adhesive if you feel so inclined. Ballast and scenery will hold it in place when all is said and done.
http://i83.photobucket.com/albums/j319/pcarrell/Autumns Ridge/Picture021.jpg
This was all layed and caulked in about ten minutes and is ready for track.
http://i83.photobucket.com/albums/j319/pcarrell/Autumns Ridge/Picture002.jpg
http://i83.photobucket.com/albums/j319/pcarrell/Autumns Ridge/Picture012.jpg
http://i83.photobucket.com/albums/j319/pcarrell/Autumns Ridge/Picture011.jpg
(This shows with the track installed, but it was part of that which went down in about ten minutes. Obviously, the track took longer.)
http://i83.photobucket.com/albums/j319/pcarrell/Autumns Ridge/7-15-077.jpg