You're amazing!" "Amazing? Not really; just built for speed, that's all!

*Caitlin is a big, magenta American streamlined tender engine with an Irish accent from the Mainland, who is designed and built for speed. She has her own rake of coaches painted in the same livery as her.

Caitlin is a streamlined engine, designed and built 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 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, United States 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 from Jersey City, New Jersey, to 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 from Baltimore to Cincinnati, Ohio. It was retired from service and scrapped in 1958.

One non-streamlined member of this class, first-of-class No. 5300 President Washington, is preserved at the B&O Railroad Museum in Baltimore.

Caitlin has been scaled down from her basis, with the rear section of her cab and doors noticeably thinner than her basis. She has also been partially modified to work on British railways with a screw-link coupling and buffers on the back of her tender.

Caitlin is painted red-violet with periwinkle and white lining. She has golden nameplates with her name written in black on the sides of her boiler.*