Bill & Ben are twin tank engines who work for the Sodor China Clay Company. They are painted yellow, have four wheels each and have their names attached to their sides on brown nameplates. Apart from their nameplates, they are absolutely identical—which can be confusing to engines who don't know them well. They are smaller than the other engines they work with. They were built in 1953 and arrived on Sodor in 1966.
They are young and cheeky and love playing jokes on the big engines, but are kept in order by Edward and BoCo. They work in the china clay quarry and at Brendam Docks, where they are kept busy shunting trucks.
Bill & Ben are based on two 0-4-0 tank engines built by Bagnall's of Stafford. They worked at Par in Cornwall and were named Alfred and Judy. They are unusually low, allowing them to fit under bridges that taller engines could not. The real examples are both preserved at the Bodmin and Wenford Railway.