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. But Derek is a likeable chap and even Bill and Ben decided that they liked him due to his friendly nature. Derek knows too, if he were fixed properly he would show what a Really Useful Engine he can be. He is also willing to help another engine in trouble.