Den is a diesel engine who runs the Dieselworks with Dart.
- Take-n-Play version (uni-polar magnets, made in China) (2011-2012)
- Take-n-Play version (minor updates, made in Thailand) (2012-2014)
- Talking Den (Take-n-Play)
- Dieselworks Fix-Up (Take-n-Play [bi-polar magnets], Adventures [hook and loop couplings])
- Den was first sold as a Target exclusive in the US before his general release.
- The 2011 packaging features a special Day of the Diesels design.