I have several "dig hole" type tunnels on my website, that you might try. There is also a tutorial describing how to install them. If they fit your needs, you'll find them on the DLS. Click on my sig below.
I'm not sure if it's on the DLS or not, but I find HP Trainz's LP Tunnels to look really nice. They have become my tunnel of choice now for many routes I've been working on.
Hi Raphaël,
Please look under my KUID (23299) on the DLS .
You will find real dark tunnels and tracks ... and some lights that fit above the tunnel niches .