Millie is Sir Robert Norramby's private French narrow gauge engine. She works at Ulfstead Castle along with Stephen and Glynn. She has worked with the Earl of Sodor 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. Unlike her basis, Millie has a rear coal bunker. "Tabamar" is preserved at the Chemin de fer des Chanteraines in France. Millie has been regauged from 1ft 11 5⁄8in to 2ft 3in.
- It is currently unknown when Millie will appear.
- It is currently unknown who will be voicing Millie.
- Millie has the same whistle sound as Lady.
- She is the first female narrow gauge engine in the series.