Bill and Ben are tank engine twins belonging to the Sodor China Clay Company. They are young and cheeky and love playing jokes on the bigger engines, but are kept in order by Edward and BoCo. They work in the china clay works and at Brendam Docks, where they are kept busy shunting freight cars.

Bill and Ben are Sodor's most mischievous saddle tank engine twins, who have great fun in teasing and playing jokes on other engines of the railway. The two have four wheels, a tiny chimney and dome and a small squat cab. They often work in the China Clay pits and quarries. Their loads of clay can be used for many things such as paper, paint and pottery. They take trucks for engines on the mainline and ships in the harbour and are regularly seen around Brendam Docks. They enjoy attention and love taking visitors and being photographed. They tend to work together, like most twins, but even this youthful pair can have their arguments among themselves and can squabble with one another - particularly about whose fault one of their follies was. They are highly prone to playing tricks and being cheeky. They have even teased Gordon on more than one occasion. Duck has called them "the bees" before, for being terrors when they start buzzing around. Edward and BoCo are the only engines who can really keep them in order enough to put a stop to their games and make them scamper along and behave. There is no real harm to them, but even Edward admits that they are maddening at times. Bill appears to be the quicker-witted leader, while Ben seems to be the follower of the pair.