- “Well, what I mean to say is...”
- ―Den thinking about what he's going to say before Dart interrupts him
In the fifteenth season, he, along with Dart and Diesel, tried to find a Christmas tree for the Dieselworks whilst in competition with Thomas. Den later teased Flynn for spraying the Fat Controller and Gordon.
In the sixteenth season, he helped Thomas fix Ol' Wheezy and repaired Gordon's bufferbeam.
In the seventeenth season, Den, with Dart, help fixed a sputter in Salty's engine.
In the nineteenth season, he was sent to the quarry as a replacement for Mavis while she was at the Dieselworks. Initially, Den struggled with the troublesome trucks, but after some encouragement from Toby and Henrietta, he toughened up and finished his job. When Mavis came back, Den went back to the Dieselworks.
In The Great Race, Den, along with Dart and Paxton, was part of Diesel's plot to be taken to The Great Railway Show. Diesel had the three put under crates in a large train, making them resemble vans. When the Fat Controller was in the final stages of choosing who would go to the show, Diesel would then honk his horn to signal the three to start pushing, making it look as if Diesel was moving the train by himself, whereas the other three diesels would actually be doing most of the work, which Diesel hoped would convince the Fat Controller to take him to the show. The day before the show, Diesel refused to move his train from blocking all ways in and out of the yard, leading Thomas, who was in a hurry to demonstrate his shunting skills to convince the Fat Controller to enter him in the shunting competition, to move Diesel's train himself. When he whistled after a shunter coupled him up, the diesels mistook the tank engine's whistle as Diesel's horn and started pushing the train as they were instructed to. Thomas was surprised at the train pushing him and Diesel was horrified, thinking Thomas was pulling the train, realising his chance would be ruined. Though Sir Topham Hatt was impressed with the heavy train Thomas was "pulling," Thomas then collided with Norman at a set of points, losing his left buffer and ruining his chance to enter the show as well. Later, Diesel confronted the three and nearly forced Dart off the Dieselworks turntable, but instead made Dart bump into a button, sending a crate down on Diesel. Diesel later escaped to Brendam Docks, and the three continued to hunt him down, only for Cranky to load him onto a ship bound for the mainland.
Den is the diesel engine who runs the Vicarstown Dieselworks, repairing other diesels. He is stately and deeply pensive. He tries to make sure everything he says is deeply considered, can often be heard muttering "what I mean to say is...", and can tend to over-think. He may come across as a little slow, but nothing could be further from the truth. He is rather astute and likes to take into account as much as he can with regards to a problem. Although he is not a worrier, he may tend to be on the fence about taking particular action over a matter before making a calculated, informed decision that will be worth the wait. Sometimes though, Dart, his assistant at the Dieselworks, can tell what he is thinking and cut to the chase a little quicker for the benefit of the other engines, which is why they support each other so well as a team. Den is a hard worker and is popular and well respected among the diesels for fixing them with a whisk of his wheels, even if his tools may be old and shoddy and his machinery may be outdated. Although he is trustworthy and authoritative, Den may have the occasional chuckle at an engine's expense.
Den is based on a Rolls Royce Class 4DH Sentinel diesel-hydraulic 0-4-0. These engines were bult to replace Sentinel's steam line for use in various industrial shunting roles. Larger 0-6-0 and 0-8-0 variants were also produced. Several engines of this type are preserved, including one at the National Railway Museum, and another at the Bluebell Railway.
Den is painted slate grey and orange with cream lining. His buffer beams are painted with orange and black wasp stripes. He has the Vicarstown Dieselworks logo on each side of his cab.
- Keith Wickham (UK/US; Day of the Diesels onwards, excluding Fiery Flynn)
- Rupert Degas (UK/US; Fiery Flynn only)
- Ryuzou Ishino (Japan)
- Fredrik Paasche (Norway; Day of the Diesels only)
- Sigbjørn Solheim (Norway; fifteenth - seventeenth season)
- Burkhard Schmeer (Germany)
- Piotr Bąk (Poland)
- Víctor Covarrubias (Latin America; Day of the Diesels onwards)
- Bernard Demory (France and French speaking Canada)
- Tiaggo Zambrano (Brazil)
- Ángel Amorós (Spain)
- Denis Bespalyy (Russia; nineteenth season onwards)
- Jukka Voutilaninen (Finland; Day of the Diesels onwards)
- Haris Grigoropoulos (Greece)
- In the nineteenth season, Den gained a standard tail lamp, despite already having two the same design as his front lamps.