Anthony,
Have a look at TRACKRITE underlay, before laying cork. It gives sound insulation and is the correct size for N gauge track. It also provides the correct spacing for double track simply by laying it side by side.
Not sure if they have a website, but it is widely available in Australia.
Last email address I have for them is mailto: jkhobbies@dragnet.com.au
I've used this extensively myself and can recommend it .
pgilbert
(Sunny Sydney)