Den is a Hydraulic-Diesel engine who works on the North Western Railway and runs the Sodor Dieselworks with his assistant, Dart.


Den and Dart took Thomas to the back shed of the Dieselworks while Percy, Diesel 10, and the other Diesels took over the Steamworks. Den 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 Sir Topham Hatt and Gordon.

He helped Thomas fix Ol' Wheezy and repaired Gordon's bufferbeam. Den, with Dart, help fixed a sputter in Salty's engine. Den, along with Dart, Sidney, and Diesel hid from Paxton, causing Paxton to search for them.

Den 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.

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 Sir Topham Hatt 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," 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.

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