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Runaway Engine is the fifth episode of the twenty-first season.
Millie is given extra work around the estate and gets agitated when Stephen and Glynn forget to clear up the garden waste. When Stephen also manages to hurt her feelings, and can't find her the next day to apologise, he thinks that she must have run away.
- Sir Robert Norramby
- The Grumpy Passenger
- Rheneas (cameo)
- Sir Handel (cameo)
- Peter Sam (cameo)
- Owen (cameo)
- Mr. Bubbles (cameo)
- The Teacher (cameo)
- The Ginger-haired Boy (cameo)
- The Welsh Bird Watcher (cameo)
- One of Dowager Hatt's Friends (cameo)
- Bob Golding as Stephen
- Keith Wickham as Glynn, Skarloey and the Grumpy Passenger
- Michael Legge as Luke
- Miranda Raison as Millie
- Mike Grady as Sir Robert Norramby
- Teresa Gallagher as Some Passengers
- A few references to the seventeenth season episode, The Switch are made.
- This is the first episode of a few things:
- Luke's first speaking role since the eighteenth season episode, Duncan the Humbug.
- The first episode since the eighteenth season episode, Samson Sent for Scrap in which neither Thomas nor any other member of the Steam Team appear at all.
- The first episode since the eighteenth season episode, Millie and the Volcano to feature a narrow gauge engine as the main character.
- Going by production order, this is the sixth episode of the twenty-first season.
- This episode's title was also the working title for Blue Mountain Mystery.
- During the close-up of Mr. Bubbles, he is holding a blue, yellow and red balloon, which represent Millie, Stephen and Glynn respectively. The blue balloon then flies away, symbolising that Millie has gone missing.
- A picture of Glynn from The Adventure Begins, a promo picture of Stephen and a picture of Ulfstead Castle from King of the Railway can be seen on the sheet that the Grumpy Passenger is holding.
- When the Grumpy Passenger walks into Stephen's buffer beam, Stephen's whole frame is shifted over.
- When Stephen and Glynn discover the garden waste, Stephen's shed doors is shut. But when Millie puffs on the turntable, the doors are open. Also, since the waste is right in front of the door, there is no way the doors can open that wide.