I finally got to do the U-Bahn Tunnel Tour back in June after waiting a bit more than two years! You have to sign up early in the year to get booked as they are limited in space and only run a few months of the year. I was actually scheduled to go last October, but due to technical difficulties, it was cancelled (the first time that ever happened to a Tunnel Tour, apparently).
In any case, it was worth the wait. You ride through the U-Bahn tunnels on this open train. It was so much fun! I took a lot of pictures, but not many are actually worth showing, but hopefully I can dig out a few. Don't forget to check out Signs, Signs for your Wednesday fix.