Nelson is a ballast tractor, who works for the Sodor Construction Company. His job is to carry construction workers and the Pack to and from work sites. As the bearer of such weighty loads, he often feels overworked, but never complains.
In his first appearance, Jack asked Nelson if he was the most important, as Patrick boasted that he was the most important earlier. Nelson told Jack that he's the most important, as he's the main source of transportation. Later, when a fierce storm raged across Sodor, Nelson and Oliver were sent to clear Henry's Tunnel.
However, Nelson soon got tired of always having to carry the other machines to and from work. He began dreaming about being carried himself. The next day, Thomas came off the rails and breaks a wheel. So Nelson was sent to take Thomas to the repair yard. When this is done, to his surprise, Percy carried Nelson back to the yards on a flatbed. Nelson had a splendid ride, and it was as magical as he had dreamed.
Nelson is a clever, intelligent, and wise ballast tractor who often feels overworked but never complains. He is proud of his paint and is happiest when he looks his best. Although he will never offer up a complaint, he is the first machine to comment on what is going wrong on the construction site.
Nelson is based on a Scammell Pioneer ballast tractor with a low loader trailer.
Nelson is painted black with cream and red lining, red wheel alloys and front beam. He has his name painted above his cab windows and has the number "10" painted on his cab doors, both in cream.