Bill and Ben are Sodor's mischievous saddle tank engine twins, who have great fun in teasing the other engines of the railway. Each has 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 main line 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. 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 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.
- Bill and Ben are also best friends with Snips and Snails.
- Tigger refers to them as "Billy boy" and "Benny boy".
- Bill and Ben will make their first guest starring appearance in Thomas the Tank Engine Gets Frozen.
- Bill and Ben will guest star in Thomas and Twilight Sparkle's Adventure with Spy Kids 2: Island of Lost Dreams.
- Bill and Ben will meet Team Robot in Team Robot's Misadventures Of Thomas & Friends: Tale of the Brave and see them again in Team Robot in Spy Kids 3-D: Game Over.
- in Mustang Fan's series, Bill and Ben are Sour Sweet's and Sunny Flare's boyfriends.
- Bill and Ben appear in The Irelanders' Adventures Series when they stowaway with Connor Lacey and his team or appear randomly.