|Mavis The Young Diesel|
Mavis is a female Diesel engine who works for the Ffarquhar Quarry Company at Anopha Quarry.
Mavis is a feisty young diesel engine who is usually found working at the Quarry. She is full of her own ideas and doesn't take kindly to advice, which can sometimes get her into trouble. A friendly Diesel, Mavis is usually the first one Thomas turns to when the Steamies and Diesels need to work together. 
Mavis was particularly fond of putting the trucks at Anopha Quarry in different places, despite Toby's protests. Toby finally lost patience and left Mavis to her own devices. Mavis, urged by Daisy (Diesel in the television series), started to ignore Toby's advice and ended up looking silly when she got stuck outside Ffarquhar.
Mavis was then forbidden to leave the quarry, but when a thaw increased production at Anopha Quarry, Mavis persuaded the manager to let her go up to the first crossing in order to shunt trucks quicker, and formulated a plan to pass the level crossing with help from the trucks. However, the plan backfired when the trucks decided to bump Toby instead, and he ended up being suspended by flimsy rails over a rushing stream and Mavis had to help. Mavis confessed, but the Fat Controller and the Manager were pleased by Mavis' courage and honesty and allowed her occasional trips to Ffarquhar.
Mavis was arrogant and feisty until being brought down to earth after her embarrassing incident at the Ffarquhar crossing. Despite this, Mavis is a reliable, honest, hard-working engine, and shows respect for the steam engines.
Mavis is based on a BR Class 04 0-6-0, fitted with additional cowcatchers and side plates so she can run near public roads.
- Teresa Gallagher (UK; Hero of the Rails onwards - US; 17th season onwards)
- Jules de Jongh (US; Hero of the Rails - Blue Mountain Mystery)
- Mavis has The Ffarquhar Quarry Co. Ltd. written on both her sides.
- Since the twelfth season, she has worked at the Sodor Slate Quarry.
- She is the First female diesel the appear in the television series