There are indoor/outdoor carpets that are very suitable as track underlay, but Nuro is right to stress the necessity of it being fibre free.
Another alternative, in the USA at least, is 'Homasote'.
This is a cellulose based sound deadening material commmonly used as underlay on model railway tracks and slot tracks.
It needs sealing first and is then paintable.
I think I first came across this material on Gregory Braun's site.
Gregory started out in HO a long time ago and branched out into 1/32 scale about 3 years ago. I cannot recommend his site highly enough for anyone contemplating table and track construction - in any scale. His site is superbly well designed and, in its field, I have no problem in rating it as simply THE best there is.
I commend everyone to take a good long look there and I feel they are bound to find it of interest. It is SO good that it could spur you into building a permanent layout though you had never before considered it!
Here are a couple of specific links, but do take a look around the whole site while you are there - leave an hour or two for this - its a large and fascinating place!