In 2011, Den and Dart took Thomas to the back shed of the Dieselworks while Percy, Diesel 10 and the other Diesels took over the Steamworks. Later, he, along with Dart and Diesel, tried to find a Christmas tree for the Dieselworks whilst in competition with Thomas and later teased Flynn for spraying The Fat Controller and Gordon.
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 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Diesels for fixing them with a whisk of his wheels, even if his tools may be old and shoddy and his machinery may be outdated.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 not devious, Den may have the occasional chuckle at an engine's expense.
Den is a Rolls Royce Sentinel Diesel-Hydraulic 0-4-0.
Den is painted slate grey and orange with cream lining. His buffer beams are painted with orange and black wasp stripes. He has the Sodor Dieselworks logo on each side of his cab.
- Den is featured on the Dieselworks logo.
- He has two working electrical lamps on top of his buffer beam, and two crankshafts attached to his side rods on each side.
- The steam engines were unaware of Den's existence until his introduction in Day of the Diesels.