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 Because of these problems, all Class 17s were withdrawn by the end of 1971, after less than 10 years of service.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.