Erin is a curious-designed 2-2-2 single tender engine. She is named after her designer's mother, Erin O'Patrick McIlwain.
Erin was built by the Sodor and Mainland Railway in 1882, originally engine number 5. When the box tanks were scrapped, Erin became number 2 in 1910. Before closure of the S&MR, she had a terrible accident in 1914, and withdrawn from service. However, in 2009, Percy discovered her, and promised to keep it a secret from Sir Topham Hatt. When found out by Alfred, Percy reluctantly told Sir Topham Hatt who understood, and now Erin is a really useful engine again and works on Edward's branchline as a mixed traffic engine.
Erin is kind-hearted and helpful to her friends, but can be very sensitive and shy. She is the only female engine who thinks James is awesome. Her best friends are Percy, Thomas and Lady.