Derek is a diesel, best known for his initial "teething" troubles.
Derek is a large diesel who's engine causes him constant trouble by overheating. Derek spends most of his time at the quarries and clay mines near Brendam Docks working with Bill and Ben. At first Bill and Ben disliked Derek, but because of Derek's kind soul, they soon became good friends. 
The Fat Controller 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 is kind and enthusiastic about his work, but unfortunately for him his problem with his cooling system prevents him from working to his full potential.
Derek is based on a BR Class 17, a Clayton and Beyer-Peacock Diesel-electric Type 1.
- In 1999, the official website referred to Derek as "Paxman Diesel".
- Derek wasn't named in his first appearance, only referred to as "the new Diesel". It was not until the release of the merchandise when his name was revealed. Originally, he was going to be named "Paxman", but it was changed for fear it would offend the Paxman brand of diesels, or refer to celebrity Jeremy Paxman.
- Derek's theme is a remix of Mavis' theme.
- Derek's problems were undoubtedly inspired by his basis, as the twin Paxman engines on the real Class 17's were unreliable, being prone to problems with the camshaft, cylinder head, and other areas.