Mavis is a female Diesel engine.
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 stuck outside Ffarquhar. Of all diesels, Mavis is the first that Thomas turns to when steam engines and diesels need to work together.
- She has worked at many different quarries.
- She was the first female Diesel engine introduced in Take-n-Play.