Bill and Ben are two tank engine twins that work for the Sodor China Clay Company, near Brendam. They enjoy playing tricks and being cheeky to bigger engines. Bill and Ben were built by W.G. Bagnall in 1953 and arrived on Sodor in 1966.
 Main Line Engines
One day, Bill and Ben returned from shunting some trucks to find that some other trucks previously shunted by them were missing. Their drivers, after examining a puddle of oil on the ground, concluded a diesel was responsible. After the twins argued over who brought the "diseasel" into the yard, the drivers suggested they go looking the diesel. The twins were horrified at the possibility that the "diseasel" might magic them away like he did with the trucks, but their drivers quickly persuaded them otherwise, and so they set off.
At last, the twins found the "diseasel". While Ben ran behind, Bill confronted the "diseasel" over the theft. The "diseasel" claimed the trucks were his and ordered Bill to "go away"; Bill pretended to be frightened and slunk away, remarking, "You'll be sorry". With that, Ben ran up and accused the "diseasel" of being a "truck stealer", before running back so Bill could run up again. This continued until the "diseasel", under the impression that there was only one very swift and annoying saddletank, became so giddy that his eyes "nearly popped out", just as the twins puffed up beside him. The "diseasel", initially confused, laughed at his ignorance. When Edward arrived moments later, enquiring as to why Bill and Ben were not shunting, the twins angrily claimed that the "diseasel" had snuck into their yard and stolen their trucks. Edward soon realised what was going on, and explained the situation. The twins, embarrassed, apologised to the "diseasel", who asked to be called BoCo, and then left to shunt some more trucks, leaving Edward and BoCo to laugh about the incident.
A few days later, after BoCo took the Express in James' "absence", Edward recounted what Gordon had said about branchlines to the twins. The next morning, after Gordon ended up at Brendam after a series of events involving poor preparations and a large floppy green hat, the twins snuck up beside him and loudly recounted what Gordon had said about branchlines. Concluding that the engine beside them could not be Gordon, the two declared he was nothing more than a pile of old scrap-iron, and began to contemplate what they should do (Bill was all for sending Gordon to be scrapped, while Ben favoured the more humane, or engane, option of dumping him in the sea), all the while ignoring Gordon's protests, until at last, BoCo arrived. Gordon begged him to save him, and so BoCo threatened the twins to leave with the trucks he had brought. The twins quickly left, leaving Gordon with BoCo. BoCo wisely neglected to tell Gordon the twins were joking.
Sometime later, a group of enthusiasts came to visit the railways of Sodor. When Edward brought them to Brendam, the twins took them to the China Clay Workings in a "Brakevan special".
 Thomas and the Twins
In 1988, Thomas was sent to work on Edward's branch line while repairs were carried out on the bridge. The twins were quick to tease him about his colour, but Edward was savvy enough to sent the twins away before an argument broke out.
After Thomas returned to the Yard, covered in dairy from a collision with a farm lorry, the two teased him even more about his new livery. This continued until Edward announced a party of enthusiasts were coming, and threatened that he would not bring them to see the twins. The twins, excited, decided instead to worry about their own paint - ie, how clean it was - although one day, when the drivers remarked that "the enthusiasts [wouldn't] eat their dinner off [them]", Bill quipped that "They might if we were Thomas".
When, at last, the enthusiasts did arrive, a photographer with an "instant" camera began harassing Ben by continuously taking pictures of him, even after the remainder of the crowd went to watch Bill tug a boat into deeper waters. At last the photographer prepared to take one final picture, just as Bill's driver arrived to ask for assistance. Ben's driver turned on the steam just the photographer took the picture; disgusted with the poor quality of the shot, he abandoned it on the floor and left. The picture was later discovered by Bill's driver after the two rescued the boat.
The repairs to the bridge took a long time, and Thomas was still at Brendam when the rainy season came. At the "Drain", a hollow in the land where rainwater collected, the water even began to come near the rails, although no one took much notice. One particularly wet day, as Ben returned from the Workings, the wind wipped the rainwater into a wave which swamped Ben, dousing his fire. His fireman went for assistance, and brought Thomas to help. While Thomas dragged Ben out, Bill took away the trucks. When at last the water subsided four days later and Bill returned to Brendam, the two made a pact to never tease Thomas again.