Looking back to the original thread, I think it would be cool if LEGO took on landscaping and put together a couple of sets that could modified for projects such as tunnels, etc. I'm sure AFOL would buy these up.
As for train tunnels, there are a lot of fun. My kids love watching the train disappear and reappear on the other side. There's also TONS of inspriation out there. For example, check out the Glacier Express in Switzerland (and bridges). http://www.affordablecruisestours.com/phot...and_Picture.jpg
The other challenge for tunnels if you do shows is transporting them. Reverse engineering them is time well spent. After making sure the tunnel entrances are the right size, figure out the curve of the track, and then how you can break it up into plates that can be easily transported. Fun challenge :-) Don't forget to make the guts accessable if you have trains going through. Something will surely go wrong at some point!