Isabel worked with Oliver and Toad on a branch line in the West Country. In 1967, the branch line was closed and Oliver's crew plotted for the trio to escape to the Island of Sodor. They had many narrow escapes, and eventually made it to Barrow-in-Furness, where Oliver ran out of coal. Fortunately, Douglas helped them escape across the bridge to Sodor and stored them at Crovan's Gate; Oliver's crew hid inside Isabel during the journey. Sir Topham Hatt then had the three repaired and repainted in Great Western colours for work on the Arlesburgh Branch Line.
The Fat Controller was already making plans to acquire an auto-coach to handle the passenger traffic on the re-opened Arlesburgh branch, which turned out to be Dulcie. The arrival of Isabel, with Oliver, to augment the stock could not have been better timed. The two were shortly after joined by Alice and Mirabel, who were paired with Duck.
Oliver says Isabel is very clever, as she always keeps a good look out when he pushes his trains and can't see where he's going, and talks to him with her bell. He finds them to be funny coaches, and belives that they would not know what to do without him. Despite what Oliver thinks, they are very faithful to him and are easily able to see when he's headed for trouble
Isabel and Dulcie are based on Great Western Railway auto coaches, which were used in push-pull service on branch lines. The auto coaches were efficient because it eliminated the need for the engine to run around to the other end of the train to turn around. 15 examples of this type are preserved.
Isabel and Dulcie are painted in the Great Western's chocolate-brown and cream coach livery. When the former first came to Sodor, she was painted in the British Railways' maroon coach livery with yellow lining. Dulcie was also seen in this livery in the 1985 annual.
- 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)