Thomas & Friends S01E17 - Percy Runs Away

Based on the story of the same name from Troublesome Engines.

The big engines have been let out of the shed after they learn their lesson, and as reward, Percy, Edward and Thomas go and play on the branch line. However, Percy forgets Edward's warning about going on the main line and is almost caught in a collision with Gordon. Percy is so startled by Gordon that he knocks off his crew and uncontrollably runs away.