Isabel escaped to Sodor with Oliver and Toad to avoid being scrapped. Upon their arrival, Oliver and Isabel were assigned to run the Little Western branch line with Duck. To fill the roster, Sir Topham Hatt rescued three more autocoaches, Alice, Mirabel, and Dulcie. While Alice and Mirabel became Duck's coaches, Dulcie joined Oliver and Isabel.
Isabel and Dulcie are based on Great Western Railway autocoaches, which were used in push-pull service on branch lines. The autocoaches were efficient because it eliminated the need for the engine to run around to the other end of the train to turn around.
Isabel and Dulcie are painted in the Great Western's chocolate-brown and cream livery. When she first came to Sodor, Isabel was painted in the British Railways' maroon livery.
- Isabel's name originates from the fact that there "is a bell" on her.
- Isabel's name is misspelled "Isobel" in the ERTL range.
- ERTL (Isabel only; maroon; discontinued)