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 Millie's Carriage 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.