Rosie is a lively, cheerful, cute little red (originally pink) tank engine. She is very kind and works really hard. She used to idolise Thomas, who sometimes got annoyed by her. After Thomas had an accident in the course of delivering his train to High Farm, she helped him and Thomas started to be friends with her. Since that day, Thomas has been her best friend ever.
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 later helped Thomas shunt trucks until Hector frightened her away.
Rosie was later 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, clever and helpful little tank engine with a good sense of humour. She idolizes Thomas very much. She and Thomas like to have fun and also like to help each other when they are working. She also enjoys racing just as much as she loves to be really useful, and is very eager to prove her worth, as evidenced by her wanting to help Thomas deliver Alice's birthday presents, or even attempting to pull a special train on her own. Rosie can be described as a feisty, energetic and free-spirited tomboy who is not afraid to do hard work.
With the passage of time, Rosie became more focused and mature, serving as a voice of reason to James' arrogance. She also stands no nonsense for teasing or rudeness, although she does have a good sense of humour and likes a friendly joke.
Rosie is best friends with Thomas, as the two of the them like to have fun together. Rosie wants to be just like him as she loves to be tomboyish. She still manages a healthy relationship with Thomas, despite some incidents that unintentionally go wrong. In Thomas Sets Sail, when the sail of the boat can’t go under the bridge, Rosie gets worried. In Thomas and the Runaway Car, when Thomas unintentionally pushed Sir Topham Hatt's Car down Gordon's Hill, Rosie was just at the wash down when the car passed by and covered her with mud which made her annoyed. In Rosie is Red, Thomas pretends not to remember sticking up for Rosie because he does not want to be teased. This ends up embarrassing Rosie. This is one of the only times where she is embarrassed.
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.
Rosie was originally painted lilac/dark pink with light grey tank panels, gold boiler bands, and red detailing on her wheels and metalwork. Starting from the twenty-first series onwards, Rosie is painted cherry red with dark raspberry tank panels, gold boiler bands and light grey lining. She has gold lining above her wheels and around her ladder. Rosie has the letters "NWR" written on her tank panels in light grey and the number "37" in light grey under her cab windows. She still retains her original red buffer beams and wheels with light grey wheel rims however.
- Season 10 - Thomas and the Birthday Mail
- Season 11 - Hector the Horrid, Thomas Sets Sail, and Thomas and the Runaway Car
- Season 12 - Rosie's Funfair Special
- Season 13 - Steamy Sodor (cameo), Splish, Splash, Splosh!, and Hiro Helps Out (cameo)
- Season 14 - James in the Dark (cameo), Diesel's Special Delivery, Merry Winter Wish (cameo), Thomas and the Snowman Party (cameo), Thomas' Crazy Day (cameo), Jumping Jobi Wood! (cameo), and Merry Misty Island (cameo)
- Season 15 - Gordon and Ferdinand (cameo), Toby and Bash (cameo), Percy's New Friends (cameo), Henry's Happy Coal (cameo), Surprise, Surprise (cameo), Stuck on You, and Kevin the Steamie (cameo)
- Season 16 - Race to the Rescue (cameo), Ho Ho Snowman (cameo), Flash Bang Wallop! (cameo), Thomas Toots the Crows (cameo), Bust My Buffers! (cameo), Percy and the Calliope (cameo), Salty's Surprise (does not speak), Sodor Surprise Day, Muddy Matters (cameo), Whiff's Wish (cameo), and Happy Birthday Sir! (cameo)
- Season 17 - Steamie Stafford (cameo), Henry's Hero (cameo), The Thomas Way (cameo), Percy's Lucky Day (cameo), Bill or Ben? (cameo), Away From the Sea (cameo), and No More Mr. Nice Engine (cameo)
- Season 18 - Signals Crossed (cameo), Toad's Adventure (cameo), Long Lost Friend (cameo), and Samson at Your Service (cameo)
- Season 19 - The Beast of Sodor (cameo), Very Important Sheep (cameo) and Philip to the Rescue (cameo)
- Season 21 - Hasty Hannah (cameo), The Fastest Red Engine on Sodor and Emily in the Middle (cameo)
- Season 22 - Forever and Ever (cameo; fantasy), Confusion Without Delay (does not speak), An Engine of Many Colours (fantasy) and Rosie is Red
- Season 23 - Chucklesome Trucks (cameo) and Steam Team to the Rescue (cameo)
- Season 24 - A New Arrival, The Great Little Railway Show (does not speak), Nia and the Unfriendly Elephant (cameo) and James the Super Engine (cameo)
- The Great Discovery (cameo)
- Hero of the Rails (cameo)
- Misty Island Rescue (cameo)
- Day of the Diesels (cameo)
- Blue Mountain Mystery (cameo)
- King of the Railway (cameo)
- Tale of the Brave (cameo)
- Sodor's Legend of the Lost Treasure (cameo)
- The Great Race (cameo)
- Journey Beyond Sodor
- Big World! Big Adventures!
- 2007 - Airship
- 2008 - Flower Engine
- 2011 - Rumble Rescue, Snug in a Rug, Market Day, Rosie's Rocks, Rosie's Really Useful Scrap!, Harold and the Kites, Flower Engine, and Diesel's Special Delivery (does not speak)
- 2012 - Stuck On You, The Fat Controller's Bedtime Story (Issue 643), Snug in a Rug, and Messy Rescue!
- 2013 - Sodor Surprise Day (speaks in speech bubble only) and Boiler Bother (speaks in speech bubble only)
- 2015 - It's Okay to be Scared
Rosie also appeared in the magazine stories, Frying or Flying, Mighty Magnet!, Really Useful Rosie, Red for Rosie, Rushing Around, and Stone Signal.
- Rosie is usually a very happy engine. But sometimes she can feel worried, annoyed or very embarrassed.
- It is unknown if she will appear in Thomas & Friends: All Engines Go! But, if she does appear, she would probably most likely be very sporty because of her tomboyish nature.