Derek is a diesel who had teething troubles.
Boco realised Bill and Ben needed help and sent for Derek, but Derek's cooling system overheated on Gordon's Hill and Boco had to come to the rescue. The news of the rescue went through the North Western Railway's rumour mill in due course, until a confused Percy informed Bill and Ben that Derek had, of all things, a toothache! Bill and Ben were apprehensive of Derek as a result, but forgot their fear when Derek's cooling system overheated yet again and they had to help him home. Derek was then sent to the Works to be mended.
- Derek has a BR logo that says "Thomas Tank" Instead of "British Railways".