Bill and Ben are cheeky twins!
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.
Despite their cheeky and mischievous antics, if a dangerous event occurs, such as a rockslide, they are shown to be quick-witted and capable to act to rescue anyone in trouble. This was shown when they saved a group of workmen and Thomas from many rockslides. The two also usually try to keep their jokes from becoming harmful, and will show remorse if their cheeky antics go too far and like all engines on the North Western Railway, they want to be Really Useful.
Bill and Ben are based on the Bagnall 0-4-0STs "Alfred" and "Judy" of Par, Cornwall, who are both preserved and in working order at the Bodmin and Wenford Railway. According to the foreword of Thomas and the Twins, Alfred and Judy are both Bill and Ben's twins.
Alfred was once repainted yellow for a Days Out with Thomas event, to resemble Bill.