Dart is a small diesel shunter who assists Den in running the Vicarstown Dieselworks.
In Day of the Diesels, Dart complains about needing a new crane and hides Flynn's hose in the rubbish by order of Diesel 10.
Later on, he went with Thomas, Den and Diesel to Misty Island in search of a Christmas tree for the Dieselworks. Dart later teased Flynn for spraying Edward and his driver.
He then helped Thomas fix Ol' Wheezy and later fixed Gordon's bufferbeam.
In the eighteenth season, he hid from Paxton along with Diesel, Den, and Sidney causing Paxton to make an island-wide search for them.
At one point, Mavis had an accident, and Den was forced to replace her at the quarry. Dart was distraught at being separated from his best friend, and was slow to start repairing Mavis. When Mavis reminded him that the quicker she's fixed, the sooner he can be back with Den. Dart took her advice and soon, Den returned.
Dart works at the Vicarstown Dieselworks to support and assist Den. Den and Dart have worked together for so long that Dart can often guess what Den is thinking before he can get it out himself. Although not so much the thinker of the duo, he is usually the doer and can be the chattier of the two. He quickly buzzes about the Dieselworks, occasionally bashing, crashing, and clanging into things. He can be heard to grumble "Dizzy Diesels!" when something goes wrong. Dart is very able in his work and a valuable team player. Despite being small in size, he is feisty and fast. Dart also like jokes, sometimes to the annoyance of the other Diesels. If feeling bothered, particularly by the steam engines, Dart is far from devious, but can get grumpy, gruff, irritable, and sometimes cheeky too, but otherwise he is the quick-witted assistant of the Dieselworks.
Dart based on 0-4-0 diesel-hydraulic shunter no. 3207 "Leys" built by W. G. Bagnall. It was the last locomotive built by Bagnall before the firm was taken over by English Electric. It is now preserved on the Foxfield Railway, along with Harvey's basis.
Dart is painted yellow and red with white lining and the Dieselworks logo on the sides of his cab. He has yellow and black hazard stripes painted on his bufferbeams.