Den is a Hydraulic-Diesel engine who works on the North Western Railway and runs the Sodor Dieselworks with his assistant, Dart when Diesel 10 isn't around.


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