I'm new to railworks too but I've succeeded in making a tunnel. There are a couple of useful YouTube videos.
The route I'm doing at the moment is single-track so I used single tunnel-track (track in a tunnel), which is linear and you lay it just like other track - it will go through a hill. What you need to do is place a tunnel decal - from the track menu - over where you want the hole in the hill - and that makes the section covered by the centre of the decal invisible but at this stage the tunnel entrance looks untidy and unrealistic. So you place a suitable tunnel entrance over it and it really works.
The decal is adjustable, if it's too big you can see 'invisibility' around the tunnel entrance - too small and the entrance is partially blocked. It's a bit fiddly first time but when you come to tidy up the other end of the tunnel you'll know how to do it!
Here's where I found out how to do it.
http://www.youtube.com/watch?v=k8z0IyNBL6M Try the different tunnel tracks an you'll find there are a couple that include the tunnel 'tube' - much easier!
You can do the same for a double track tunnel except you have to lay the double track tunnel and its track separately, I think.