*Rosie is a lively little tank engine who lives and works on the Island of Sodor, and is the North Western Railway's number 37 engine.

Rosie is a friendly, clever, feisty and energetic tomboy with a good sense of humour. She is free-sprited, quick, and loves having fun. Rosie enjoys racing, surprises and playing the "Splish, Splash, Splosh" game in particular. She used to idolise Thomas and would attempt to imitate him, but has since become more independent while maintaining a good friendship with Thomas, despite some incidents that unintentionally worry, annoy, or even embarrass her. Rosie likes hard work and being really useful, and is very eager to prove her worth.

Though she may appear childish at first glance, Rosie is much stronger and more capable than she appears. She is also a very determined and dependable engine, who will always go the extra mile to help her friends out when they are in a spot of bother.

With the passage of time, Rosie became more focused and mature. She stands no nonsense for teasing or rudeness towards others or herself. Although she is now a much wiser engine than when she first arrived, she retains her tomboyish personality, good sense of humour and enjoyment of racing.

Rosie is based on the Southern Railway (SR) USA Class. The class originated as the United States Army Transportation Corps (USATC) S100 Class, which were designed and built for shunting duties in Europe during the Second World War. After the war ended, fifteen of the S100s were sold to the Southern Railway, who modified them for use as dockyard shunters at Southampton. There, they worked alongside the London, Brighton and South Coast Railway (LB&SCR) E2s (Thomas' class). The SR USA Class was replaced by British Rail (BR) Class 07 diesels (Salty's class) in 1962, however some members of the class remained in service on departmental duties until 1967.

Four members of SR USA Class are preserved, including one at the Bluebell Railway. Over one-hundred of the USATC S100 Class - of which the SR USA Class are a derivative of - still remain, or are preserved, in various conditions in some European countries, the United States, China and Egypt. In addition, several Yugoslavian-built copies of the S100 design also exist, including two that have been shipped to England and modified to represent USA Class members.

Unlike her real-world basis, Rosie's main rods from her cylinders connect to her middle set of driving wheels instead of the back ones. Additionally, her whistle is situated between her cab and rear sand dome, rather than on the side of her steam dome.

Starting from Journey Beyond Sodor onwards, Rosie is repainted cherry red with dark raspberry tank panels, gold boiler bands and light grey lining. The area above her wheels and around her ladder are painted gold. Rosie has her company's initials (NWR) painted on her tank panels in light grey with a gold shadow, along with her number 37 also in light grey with gold shadow under her side cab windows. She still retains her original red buffer beams and wheels with light grey rims, however. Her makeup had also been removed.

The font style of Rosie's NWR initials and number 37 is the same font that is used on fellow SR USA Class, Vulcan 4446, now No. 30072 of Keighley and Worth Valley Railway.*