Rosie is a tank engine who runs her own branch line.
Rosie began idolising Thomas to the point of hero-worship and imitation. Though this is said to be the most sincere form of flattery, it only served to irritate Thomas whilst he was trying to deliver a special birthday post train to Alice at High Farm. He even started to try and avoid her. After Thomas had an accident in the course of delivering this train, she helped him and he started to respect her. Sir Topham Hatt rewarded Rosie by putting her in charge of the Ulfstead Branch Line.
Rosie later helped Thomas shunt trucks until Hector frightened her away.
Rosie was later charged with acting as back-engine to Emily with the funfair special. Wanting to take the train herself and, thinking she was doing Emily a favour, Rosie succeeded only in creating confusion and delay on a grand scale, but eventually set things right.
Rosie rescued Thomas when his fire went out after playing a game of "splish, splash, splosh". She later let Diesel take her train of flowers which was headed for market.