Along with Connor, Thomas, Percy and James, she helped with the search of Stephen before Ulfstead Castle's opening. Caitlin once had to spend the night on Sodor and became too excited to listen to Sir Topham Hatt's advice. During her night on Sodor, she took the Mail Train and the Flying Kipper. Unfortunately, she did not take others into consideration and ended up waking lots of people up.
Once Annie and Clarabel, who were in the yard waiting for Thomas, were accidentally coupled up to Caitlin, thanks to Charlie. She even took them to the Mainland, but was stopped by Hiro, who brought them back to Thomas.
Caitlin later thought that Toby was going to be scrapped after hearing about it from Gordon. Later on, she took Emily to the Steamworks after she broke down - Emily later saved Caitlin when her brakes failed and she became a runaway.
Caitlin is a streamlined engine, designed for speed. Along with her friend Connor, Caitlin is an excitable and energetic character, who likes to be busy and loves to race. Both Caitlin and Connor are friendly and eager to get other engines to join in the fun with them. Caitlin is particularly impatient and restless if she has nothing to do. She would rather be racing than sit in a station and be idle. Caitlin and Connor were introduced to the island to bring passengers from the Mainland. They zip on and off the island frequently using the Vicarstown Bridge.
Caitlin is based on the Baltimore and Ohio Railroad streamlined President P-7 Class. Her basis was built by the Baldwin Locomotive Works in Eddystone, Pennsylvania, USA in 1927. The member of the class that Caitlin is modelled after, No. 5304 President Monroe, was fitted with a streamlined shroud designed by Otto Kuhler and classified as the P-7a in 1937 for use on the Royal Blue train service between New York City and Washington, D.C. Its streamlining was removed in 1940 and it remained without streamlining until 1947, when it, along with three other class members was re-streamlined in a different design and re-classified as the P-7d class for service on B&O's Cincinnatian train service between Cincinnati and Detroit. It was retired from service and scrapped in 1958. One non-streamlined member of this class, 5300, is preserved at the B&O Railroad Museum in Baltimore.
Caitlin is painted fuchsia with light grey and light blue lining. She has golden nameplates on the sides of her boiler.
- Rebecca O'Mara (UK/US; King of the Railway - nineteenth series; The Great Race)
- Jules de Jongh (UK/US; twentieth series only)
- Merete Brettschneider (Germany)
- Ewa Serwa (Poland; King of the Railway - eighteenth series)
- Marta Dylewska (Poland; nineteenth series onwards)
- Sakura Yoshioka (Japan)
- Talya Barkay (Israel)
- Sarah MacDonald Berge (Norway; King of the Railway only)
- Lena Meieran (Norway; seventeenth series only)
- Laurence Sacquet (France and French speaking Canada)
- Mireya Mendoza (Latin America; King of the Railway onwards)
- Tillie Bech (Denmark)
- Mercedes Espinosa (Spain)
- Elise Langenoja (Finland; King of the Railway onwards)
- Vina Papadopoulou (Greece)
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- Caitlin has been partially modified to work on British railways with a screw-link coupling and buffers on the back of her tender. She retains a streamlined cow-catcher (also known as a skirt) on the front, so she is unable to shunt.
- In the twenty-first series, Caitlin gained a tail-lamp.