Den runs the Sodor Dieselworks with his assistant Dart, he works at the Dieselworks fixing Diesels, Dart is Den's helper, he can read his mind. He is very thoughtful and thinks hard before he speaks. When Den does speak, it is very slowly. This gives the appearance that he may be a little slow, but nothing could be further from the truth. Engines can wait a long time for Den to give his insight, but it is worth the wait.
- "Tree Trouble" at the Dieselworks
- Den, despite being on the newest additions to the line, is the first Diesel to be coloured blue with a regular face (D199 has a yellow face).