Millie is a bubbly French engine who is owned by Sir Robert Norramby, and who runs the narrow gauge line at Ulfstead Castle, where she lives and works. Her duties include assisting the groundskeeper and taking visitors on tours of the estate. For this, she has her own open-topped carriages. While the Earl was travelling the world, Millie stayed in her shed at the castle.
Millie once swapped jobs with Luke. Although she struggled to adjust to start with, she ended up loving the Blue Mountain Quarry, although she was pleased to return to her own job at the castle and even more pleased to have made friends with Luke.
While she was helping with construction of the Dinosaur Park, she was teased by Samson and Harvey for being afraid of the Megalosaurus model. She decided to pay them back by asking the groundskeeper to make a bonfire by the model volcano to create the illusion that it was actually active.
When Glynn arrived on the Estate Railway, Stephen became jealous of the Coffee Pot, Millie, however, was more welcoming. When Glynn asked her what the Earl was planning, she suggested he was planning on building a railway museum. After Stephen stated that he would would conduct tours while Glynn would become an exhibit, Millie joked the opposite would be true. After the two old engines decided to have a race, a concerned Millie decided to tell the Earl. The Earl met the two at Knapford, where he confirmed Millie's theory that he was building a railway museum.
Once, when Millie was given extra work and spent the day collecting gravel from the Blue Mountain Quarry, Stephen neglected to do her regular chore of moving the garden waste. Fed up, Millie decided to teach Stephen a lesson by dumping the waste in front of his shed, only to get even crosser when Stephen mocked her in retaliation. The following day, Stephen could not find Millie, and worried that she had run away because of his teasing. Stephen left the estate to look for Millie, but ended up running out of water near Crovan's Gate. Luckily, Millie and the Earl found him, and explained that they had only left to pick up a new motor for the castle turntable. Millie and Stephen made up their quarrel, and, once Stephen had taken on more water, headed back to the castle together.
Millie is a narrow gauge engine. She has worked on the Earl of Sodor’s Estate Railway for many years. She is very loyal to the Earl and her relationship is like that of a personal servant. They have fun together and are as close as many friends, but there is also a certain formality to their relationship. Millie is a youthful, fun-loving, friendly engine who is full of spirit, and can be fiery when crossed. But she is also fair-minded and never spiteful. That said, she will take it upon herself to teach an uppity engine a lesson, should she feel the need. She always wants to do the best by the Earl and will defer to his judgement in all things, but when it comes to other engines, she has a mind of her own.
Millie is based on 0-4-0 well tank No. 8069 "Tabamar", built in 1911 by Borsig, under contract from Decauville. "Tabamar" is preserved at the Chemin de fer des Chanteraines in Paris, France. In 1995, Tabamar visited the Ffestiniog Railway, where it served on drive-an-engine duty at Boston Lodge. Unlike her basis, Millie has a rear coal bunker and has been regauged from 1ft 11 5⁄8in to 2ft 3in.
Millie is painted French blue with white lining, brass fittings, a golden running board, boiler bands and wheel spokes. Her wheel rims are also French blue. Sir Robert Norramby's crest is painted on her cab sides and her name is painted in white on the sides of her boiler.
- Miranda Raison (UK/US)
- Teresa Gallagher (DC Super Friends™ Minis Mash Ups Origin Story! only)
- Sonja Stein (Germany)
- Anna Gajewska (Poland; King of the Railway - seventeenth season)
- Beata Jankowska-Tzimas (Poland; eighteenth season onwards)
- Minako Saito (Japan)
- Talya Barkay (Israel)
- Sarah MacDonald Berge (Norway; King of the Railway only)
- Marit Berg (Norway; seventeenth season only)
- Blanche Ravalec (France and French speaking Canada)
- Annie Rojas (Latin America; King of the Railway onwards)
- Tillie Bech (Denmark)
- Paqui Horcajo (Spain)
- Susa Saukko (Finland; King of the Railway onwards)
- Kim Eun-a (South Korea)
- Vina Papadopoulou (Greece)
- Millie is the first female narrow gauge engine in the series.
- Millie is incorrectly portrayed as an 0-4-2 saddle tank engine in Thomas Land US.
- Millie is the first French-speaking engine in the series.
- In the twentieth season, Millie gained a tail-lamp.
- Wooden Railway
- TrackMaster (cancelled)
- Take-n-Play (normal and dusty)
- Wind-up Trains
- Classic Minis
- Robo Minis
- DC Super Friends Minis (as Bumblebee, Harley Quinn and Catwoman)
- Warrior Minis
- SpongeBob SquarePants Minis (as Sandy Cheeks)
- Construction Minis
- Night Time Minis
- Animal Minis (peacock)