Talk:Pierce Brosnan's "Thomas & Friends" Season 12 Narrations (2008)/@comment-4972669-20140721195616/@comment-34498490-20190712134100

The episode Thomas Puts The Brakes On doesn't have any tunnels in it. What you described sounds like a part of The Great Discovery, where Thomas was going through an abandoned mine and then when it ended, he jumped over a waterfall.