The Norfolk born star lent her vocals to Sodor's first French engine in the episode The Switch which aired today on Channel 5 milkshake! in the UK. Her name was listed in the end credits.
Millie is due to make her first proper appearance in the King of the Railway film later this year.
Raison is most recognisable to audiences as Tallulah in the Doctor Who episodes Daleks in Manhattan and Evolution of the Daleks and in the multi-award winning film My Week with Marilyn as Vanessa.
Miranda has also gained roles in the BBC's Spooks, Death in Paradise, Vexed and more recently as Dr. Stella Drew in ITV's Lewis.