Millie is a small, blue, French engine privately owned by Sir Robert Norramby who runs railtours at Ulfstead Castle.
Millie arrived on Sodor to shuttle around Sir Robert Norramby, the Earl of Sodor, as his private engine.
One day, Luke was visiting Ulfstead Castle when he encountered the French engine. They agreed to switch jobs for the day, and even though it was nice to have a change, Millie thought it much better to be at the Castle rather than the quarry.
She had a very interesting encounter with Wendy the incline which involved her being pulled up the steep slope.
Millie has been shown to have a competitive spirit, shown especially with her good friend Luke. She is small and cute, but she can a bit overwhelming at times.
- Season 2: Luke and Millie's Blue Mountain Adventure, Aura of Menace (cameo), All the World's a Stage (cameo), Swashbuckler (does not speak), Thomas and the Fortune Teller (does not speak)
- Season 3: Samson, BoCo, and the Man in the Hills (cameo), Culdee Fell (cameo)
- Web Clips: Mike and Frank Banter: SLOTLT, ScarletFire, and EE93's Return (cameo), Thomas the Tank Engine 70th Anniversary Celebration! (cameo)