Thomas the Tank Engine - Train lost in a tunnel

For years, a memory of an episode of Thomas the Tank Engine has lingered in the back of my mind. I tend to have a very good memory of things, but for the life of me i cannot remember the exact premise of the storyline or plot of this one episode in particular. I could try if i wanted to, but it might direct me to false leads.

My main memory of this one episode was of a train, forgotten in a sort of tunnel, lost if you will, left over time and never retrieved again. I remember a very clear scene of moss and flora growing over the spot in which the train was left and forgotten, and as a young child it left the most eerie and unsettling feeling. To be forgotten and have no one looking for you, especially in plain sight, was a thought that disturbed me as a child. I'd love to revisit this episode to get the exact memory of where im coming from.

If this helps, this aired on Treehouse TV, a Canadian childrens network. I feel it may have played later in the evening, and could've been between the years 2006-2008.