Yeah I basically made prefabs for different tunnel sections - straight, curved, one that goes up and down, a long S and so on. Then I built a tunnel junction for 4 tunnels to meet in one room. I did the whole thing with terrain patches. Once I got the tunnels aligned to the grid it wasn't too hard. Now I need to build a few more junctions to use. And yeah, Blackjack Johnny's tuts on YouTube are priceless stuff.
Tunnel Junction:
I still have some texture work to do on the junction - it looks a little too boxy to me.
The map I am working on is basically three hills/mountains with a road/path running through the middle, and a tunnel system under it:
I have one more hill to build, then I have to go through the process of placing the rest of the entities and models, and put some cover (boulders, etc.) in.