Three Steam Engines Gruff Thomas the Tank Engine & Friends : Season 20 Episode 22

One night, Percy is frightened by a strange sound from under the bridge at the Watermill. Toby is worried, though Thomas does not believe it to be true, until he too hears the sound. Toby and Percy believe it is a troll telling the story of how three goats had to cross a bridge with a troll under it. All three engines become terrified of passing the bridge until it is revealed the sound was only coming from a cow stuck under the bridge.