"Granpuff" is a fourth season episode.
Plot
One cold winter night, Percy, Toby and Duck want to hear a story, so Thomas tells them the story of Duke . . .
Years ago, three engines named Duke, Falcon and Stuart lived on a narrow gauge railway. Duke, the oldest, was wise and kept the others in order, but Falcon and Stuart teased him continuously until Duke told them about Smudger, a careless engine who often derailed until he was turned into a generator. Falcon and Stuart behaved after that, but then their line closed and Falcon and Stuart were brought by the Skarloey Railway. Duke, however, was abandoned in a shed, and as the seasons passed, Duke's shed was covered in plants and earth. Duke had fallen asleep and was blissfully unaware of this.
Percy complains that it wasn't a happy story, but Thomas replies he'll finish it some other time.