It can be found on this site -
http://www.vias3d.net/web/
There are several different kinds available, but make sure that you download the 1435mm versions, and NOT the 1668mm ones, as the UK gauge is 1435mm.....using the 1668mm track will put the wheels of your UK trains in the central ballast of the track!
You need to register to the site in order to download stuff. Then, work your way through the links on the left hand side, and eventually you'll find what you want. Register with a Username (top box) and then your email address (bottom box) and they will then send you a secret password to your email address, which is obviously worth saving somewhere.
Then, simply log in to view the content which can be found in: "Descargas > Trainz". Download as applicable
Cheers,
Ben.