'Arry and Bert are two gruff diesel twins who work for the Sodor Ironworks at the smelters.
'Arry and Bert tried to scrap a lonely Stepney, who had mysteriously been sent to the smelters by mistake. They shunted Stepney into the larger smelting shed underneath a giant grabber and left him there to be scrapped. Thankfully, Stepney was saved by Sir Topham Hatt at the last second.
'Arry and Bert are also known for playing tricks on steam engines, such as making them "middle engines". Once, they shunted trucks in front and behind Percy. This later led Percy to being pushed onto the coal tipper by mistake. When James took over Percy's duties following the incident, 'Arry and Bert played the same trick on him. Some time later, they gave Fergus, who was tricked into going to the smelters by Diesel, such a fright that he ran away.
Sometimes, 'Arry and Bert's teasing ways backfire. Once, they played a ghostly prank on Thomas and Emily during Halloween, though they ran away when they thought Emily was a ghost. Later, Diesel, 'Arry and Bert began teasing Thomas for being smelly, but were later covered in stinky cheese. By the time they had a washdown, they, along with Diesel, were assigned the task of shunting Thomas' delivery of stinky cheese while Thomas gets a washdown, much to Thomas' delight.
'Arry and Bert were also part of the group of diesels that took over the Sodor Steamworks. The pair teased Henry because of the coal he had to use by calling him "Mr. Special Steam". They also laughed at Gordon after he ran into Diesel's flatbed and damaged his bufferbeam.
Diesel claims that his friends, 'Arry and Bert, like him because he is rough and tough. Thomas attempted to blackmail Diesel into being nice to everyone with the threat of telling 'Arry and Bert of his secret affection for a group of ducklings. When Diesel broke this promise, Thomas took 'Arry and Bert to the ducklings while Diesel followed behind them. Although Thomas had intended for this to embarrass Diesel, he soon found out that 'Arry and Bert also took a liking to the ducklings and are just as big softies as Diesel when it comes to them.