Molly is a bright yellow tender engine.
Molly is a large, powerful and bright yellow engine. She's proud of her strength, but Molly is also very sensitive. She wants the other engines to think the best of her. She finds any conflict very upsetting and steams away to avoid it. 
Molly was brought by the Fat Controller to pull empty trucks to the coaling plant. Some of the other engines teased her about that, she didn't feel her job was important enough, and this made her sad. Thomas tried to cheer her up by making everyone think that she had an important train to pull, but at the last minute everything went wrong. In the end, Molly realised how important her job really was, and she knows now that pulling empty trucks can be just as important as pulling full ones.
When Emily had to work with Whiff, Molly was one of a number of engines who laughed at her. Later, Molly helped James with the removal of pipes on the line during a busy winter and then, she advised Thomas to slow down when he was taking a sailing boat for the Mayor of Sodor.
Molly is the sensitive type. Her feelings are easily hurt, and she gets embarrassed very easily, but she can have the courage to do things right.
Molly is based on the Claud Hamilton 4-4-0 of the Great Eastern Railway. Claud Hamiltons were built at Stratford Works around 1904.
Although Molly keeps the GER standard sized 7-foot driving wheels, she has a large 4' 9" boiler, Schmidt superheaters and a Belpaire firebox. Her cab is fitted with two side windows. She has a GN-style chimney, a flat firegrate and a variable blastpipe.
- Molly appeared with a larger face in promotional shots in certain Thomas and Friends magazines. This is assumed to be a prototype face, as it was never used in the television series. The face was also used on her Take-Along card and all merchandise lines thus far.
- Molly's whistle sound is really Thomas' in a lower pitch.
- She does care if Mr. Bump gets hurt.