- "I'm a survivor. You'll see."
- ―Derek, Munitions
Derek is a kindhearted green diesel who has been known to overheat and break down.
Derek first came to Sodor when work at the China Clay Works became too much for Bill, Ben, and BoCo to handle, so was bought in order to help around. However, due in part to the problems of his design (the Class 17s were BR's least successful engine), he overheated twice on his first few days on Sodor, and had to be rescued by BoCo, as well as Bill and Ben when he was their banker for a train.
He was repaired numerous times at Crovan's Gate Works, but his cooling and engine troubles continued to plague him, and The Fat Controller considered sending him back to The Other Railway during one of his repairs, and he became apprehensive about ever being able to work properly, though Paul the Mechanic and his crew often reassured him.
During this time, one diesel, D199, was carrying a goods train near where Derek was being mended, and overheated himself. Derek, who had been mended properly to move, came nearby while a solution to the matter at hand was being thought of. Derek offered to pull Spamcan's train, despite initial protest from an inspector. During this occasion, Derek was able to have a good time pulling the train, but when on Gordon's Hill, he began to overheat again, but managed to stop in time to give his engine a rest. He was then able to conquer the hill and as a reward for managing to pull the train, was allowed to stay on Sodor, officially being stationed at Crovan's Gate and helping around the island. Since this incident, Derek has not overheated as much as he did in the past, possibly due to a remedy to his probelms slowly being found.
On a cold and snowy morning at The Lumberyard, Derek and Hank were resting at a shed on site. Feeling chills, Hank tries to encourage Derek by professing his own love of the current weather with a toot of his whistle. Derek is unintentionally covered in snow from the blast of Hank's whistle, leaving the Paxman colder still. The two were able to laugh off the situation however.
When James and his driver suffered through a bout of illness and "high" (James in the latter case), Derek informed James about what had happened during the former's "trip". In addition, when Percy's anger at his increasing workload came to a head, Derek commented that an engine with all of his experience would know how to handle the trucks that Percy was pulling at the time. He also reprimanded Bear when the latter had pretended to break down in an attempt to make Fergus feel useful, stating that Bear had essentially assumed that Fergus was a weak engine.
Derek was also one of the diesels assigned to help out with munitions work, and pretty much managed to keep his cool during Diesel's attempt to take control of the job. He was surprised at Arthur coming into the yards without permission, but when a fire started due to Walter Richards, he didn't really complain and tried to escape the flames along with Diesel and Norman.
However, his cooling system gave way once again, and Diesel had Derek abandoned (although Norman wanted to help, Diesel said that if they had wasted precious seconds helping to tow Derek, they would've all been trapped). The Thin Controller helped to get Derek's driver to safety, while BoCo and Edward came to help rescue Derek themselves. BoCo used himself as a battering ram to destroy a blockage of wreckage, allowing Derek and Edward to escape. Derek called BoCo a good diesel and a good friend, saying that if it had been BoCo's end, he would've always been remembered for those traits (though BoCo survived).
When Bear felt down about what had happened with Fergus, Derek tried to comfort and consult him, and was pleased to find out that Bear and Fergus had patched up what had happened between them previously.
Later, Bill and Ben lied to him about a recent landslide, causing him to crash into the rubble, twisting his wheel frames and getting him sent away for repairs.
After returning from his repairs, he tried to comfort a put-upon Donald about the latter's workload, but when Bill and Ben came along, he quickly rushed off. It was later revealed that Derek had had a talk with Diesel shortly after his repairs, where he once again listened to Diesel's tirades. However, he did acknowledge that Diesel had a point regarding Bill and Ben, but Edward's arrival to inform them of a meeting caused this conversation to end. Derek's accident involving the twins was a factor in Edward assigning Marion as the twin's permanent chaperone.
Derek was sent to Crovan's Gate Works to collect Culdee following his repairs. During their journey to Kirk Machan, Derek struck up a conversation with Culdee about his fascination with the mountain railway, admiring his inherent bravery, to which Culdee responded that bravery is a very relative term. Upon having Culdee unloaded, Derek met Ernest, another mountain engine, whom he entered into a conversation with about being two faced, which managed to unnerve Derek to a great extent. This helped fuel his skepticism about the events happening around the island, as well as Diesel's words of the steam engines receiving better treatment and ultimately, he decided to try to see the bigger picture, and to figure out everything that was happening around the railway as of late.
Derek is polite and courteous, as well as ruthlessly optimistic. He is always willing to help out a friend, and always looks on the brighter side of things, even in a tough situation. He is a humble, cheerful sort, and often goes out of his way to keep a good eye on the well-being of those around him. Despite frequent complications with his cooling system, he does not let it ruin his mood, and calls himself a "survivor" for dealing with these problems.
However, while he is an optimist, Derek is not naive. While he was somewhat swayed by Diesel's warnings of engine favoritism and that the time had come for change, he decided not to rationalize what had been told to him by both Diesel and Edward, deciding to figure out the bigger picture and keep his guard up.
- Season 1: Lift Bridge (original version cameo), Dodge (cameo), Redemption, Feeling Lucky? (does not speak), Snow Blind (remastered version only), Buffer Bashing (cameo), Scruff's Scaffolding (cameo), Will Power, Conspiracy Theory (cameo)
- Season 2: Scot-free (cameo), Mavis and the Tornado (cameo), Rosie (cameo), Young Tucker (cameo), James Goes on a Trip, Rock-Star, Pummeling Percy, Tag-Team, Munitions, Hibernation, Henry and Kurt, All the World's a Stage (cameo), Swashbuckler (cameo)
- Season 3: Marion Saves the Day, This Too Shall Pass, Perhaps He's Got a Corset (mentioned), Culdee Fell, Derek and the Two Faced Engines, Percy and Asbestos (cameo), James Goes On A Streak (does not speak)
- Web Clips: Gina gives Percy some pointers - Meet the Characters! (cameo), Percy and Asbestos (cameo)
- Derek's old model was replaced with the 2007 model from Young Tucker onwards.
- Derek is one of the few frequently recurring characters to retain one voice actor throughout the entire series.