Millie is Sir Robert Norramby's private French steam engine.
Millie is a bubbly French engine who is owned by Sir Robert Norramby and runs on narrow gauge lines. She lives and works at Ulfstead Castle where her duties include assisting the groundskeeper and taking visitors on tours of the estate. For this, she has her own open-topped carriage. While the Earl was traveling 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 when they thought she was a silly little engine. She decided to pay them back by asking the groundskeeper to make a bonfire by the volcano to create the illusion that it was actually active.
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 the second character to speak a different language, the first being Victor.
- Millie has the same whistle sound as Lady.
- She is the first female narrow gauge engine in the series.
- While Millie and her basis have rear buffers, they do not have any on their fronts.
- Millie is incorrectly portrayed as a saddle tank engine in Thomas Land US.
- Millie is also best friends with Lyra Heartstrings.