- "Hey you two. This new diesel's got a toothache. Good luck!"
- ―Percy, Double Teething Troubles, fifth season
Derek is a diesel who is known for his initial "teething" troubles.
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 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 returned later in Calling All Engines!, pulling a goods train up Gordon's Hill.
Derek is a jolly, enthusiastic diesel engine, who was brought in to assist Bill, Ben, and BoCo at the China Clay quarry. Unfortunately, due to his engine, he is highly prone to teething troubles, namely overheating and suffering frequent breakdowns, for which he is apologetic for the inconvenience. As a result, he spends many a day having his engine tended to. But Derek is a likeable chap and even Bill and Ben decided that they liked him due to his friendly nature. Derek has an infectious charisma, is keen to help anyone around him, and has a passion for sorting things out in a jiffy - if his cooling system lets him.
Derek is based on a BR Class 17, also known as the Clayton Diesel-electric Type 1. Derek's problems were undoubtedly inspired by those of his basis, as the twin Paxman engines of the real Class 17s were unreliable, being prone to problems with the camshaft, cylinder head, and other areas. Because of these problems, all Class 17s were withdrawn by the end of 1971, after less than 10 years of service. One Class 17, D8568, is preserved at the Chinnor and Princes Risborough Railway in South Oxfordshire.
Derek is painted in the British Railways Brunswick green livery with yellow warning panels. His window frames are lined white and he has the BR "Ferret and Dartboard" crest on his sides.
- Hisao Egawa (Japan)
- Starting from the fourteenth season, Derek's horn was reused for Salty. Before this, Derek's horn was also used for the foghorn and as Mavis' horn in the sixth season episode Buffer Bother.
- Derek was the first non-Awdry standard gauge engine to be introduced into the television series.
- Derek was not named in his appearance in "Double Teething Troubles": he was only referred to as "the new Diesel", or simply Diesel and in the music video Trying it says "Who's that straining up the hill?" rather than Derek's usual name. He was only named in merchandise ranges.
- Originally, Derek 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.
- In addition, the 1999 Official Website referred to Derek as "Paxman Diesel".
- The British Railways logo on his ERTL toy reads "Thomas Tank" instead of British Railways.
- In his Wooden Railway prototype, Derek is seen with a greyish engine, unlike the black one presented in his final toy. His 2007 reissue sports the correct yellow engine.
- His ERTL prototype shows him with a darker and different face and a darker colour scheme. His 2nd ERTL promotional artwork incorrectly depicts him with steam engine type wheels, a darker colour scheme, a grey undercarriage, square buffers and with no red buffer beam.
- "You do look glum!" sighed Duck.
- "It's the new diesel's fault," replied Bill. "He's got toothache!"
- "Toothache? He's got teething troubles. That means he's new and is causing some problems. In his case it's in his cooling system."
- Duck, Bill and Ben talk about Derek, "Double Teething Troubles", fifth season.
- "Hello! I'll soon sort this train out. You pull from the front and I'll push from the back. What fun."
- Derek when he arrived to help Bill and Ben with their train, "Double Teething Troubles", fifth season.
"Come on, push harder you silly diesel!" -Ben presuring Derek, "Double Teething Troubles", fifth season.
"I'm overheating again." -Derek overheating, dragging the train to stop,"Double Teething Troubles", fifth season.