Chelsea is an LNER Gresley Class K3 tender locomotive. She was built by the Great Northern Railway in 1920. During the Second World War, she and her good friends Molly the D16 and Gordon the A3 were used for taking British soldiers to battle. Some of Chelsea's class was re-built by Edward Thompson in 1945. In 1950, Chelsea was allocated to the eastern part of Sodor, before being withdrawn in 1961 due to bad condition. In 2008, she was found by Thomas and Percy, and was given an overhaul at the Sodor Steamworks, and emerged with a new coat of NWR green, the number 16 on her cab side and the letters NWR on his tender side.

Chelsea now works on the mainline, hauling the express with Gordon, Henry, Spencer, and Bear, but sometimes works on the Little Western, and has found Duck the 57xx tank engine a great Western boyfriend.

Chelsea has a lot to learn, but she has a big heart, and feelings for Duck. Chelsea's best friends are Thomas, Emily, Molly and Gordon. She is a mixed-traffic female engine. She is one of the youngest engines, and does express runs on the main line. Young, cocky, but well-meaning, she wears her LNER green livery with pride. Chelsea shows feelings for Duck. Her best friends are Emily, Molly, Percy, Gordon, James, Henry, Duck and Thomas, and she idolizes Gordon. Her enemies are 'Arry and Bert.

Chelsea is based on the LNER Class K3.