Den runs the Sodor Dieselworks with Dart. when Diesel 10 is not around, Den runs the Dieselworks, 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. Den is not part of The Smelter's Yard like Dart and some other diesels.
Den is a Rolls Royce Sentinel Diesel-Hydraulic 0-4-0.