- "Hello, Thomas! What are you doing?"
- — Rosie, Thomas and the Birthday Mail, tenth season
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, but after Thomas had an accident in the course of delivering this train, Rosie helped him and he came to respect her.
Rosie was later called to help Thomas shunt trucks until Hector frightened her away.
Rosie was charged with acting as a 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.
One day Rosie had a cracked wheel, so Thomas and Butch brought her a new one.
On Sodor Surprise Day, Thomas bumped some coal trucks to surprise Gordon, but ended up surprising her instead. She is later seen being cleaned up with Whiff at the washdown, and joins Gordon and Thomas at Callan Castle to see Harold decked out in bright lights to make Gordon laugh.
Rosie is a friendly, respectful, and helpful tank engine. She and Thomas like to have fun and also like to help each other when they are doing work. Rosie could be described as a feisty, energetic tomboy who is not afraid to do hard work.
Rosie is an SR USA Class 0-6-0T. Fifteen of these engines were sold to the Southern Railway where they were used as dockyard shunters at Southampton, where they worked alongside the LB&SCR E2s (Thomas' class). Both classes were replaced by BR Class 07 diesels (Salty's class).
Four members of this class are preserved, one at the Bluebell Railway.
From her debut up until The Great Race, Rosie was painted lavender with light grey tank panels, gold boiler bands, red lining and wheels, and light grey wheel rims.
Starting from the twenty-first season onwards, Rosie will be painted faded cherry red with raspberry tank panels, gold boiler bands and light grey lining. She will have the letters "NWR" written on her tank panels in light grey and the number "37" in light grey under her cab windows.