- Pah! I could handle those diesels with bent buffers and a busted boiler!"
- — James, Middle Engine, sixth season
In their first appearance, 'Arry and Bert crept alongside 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 threatened to scrap him. Thankfully, Stepney was saved by the Fat Controller, once the diesels had left.
Despite being villainous, '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. Later, 'Arry and Bert played the same trick on James. Sometime later, they frightened Fergus away from the smelters, and bullied Neville, which made the engines think he was planning to bump them.
On some occasions, '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. Another time, 'Arry and Bert teased Percy for having a whistle. Percy paid them back by creeping up behind them and blowing on his whistle. Later, along with Diesel, 'Arry and Bert began teasing Thomas for being smelly, but were later covered in stinky cheese.
'Arry and Bert were also part of the group of diesels that took over the Sodor Steamworks. They later teased Henry because of the coal he had to use by calling him "Mr. Special Steam". They also laughed at Gordonafter he ran into Diesel's flatbed and damaged his bufferbeam.
In the twentieth season, 'Arry and Bert liked being rough and tough along with Diesel, they realised Diesel was trying to be nice to everyone. So Thomas took 'Arry and Bert to the ducklings while Diesel followed behind them. Diesel, 'Arry and Bert then realised the ducklings were cute.
'Arry and Bert are unquestionably devious and scheming. They feel superior to steam engines and, working at the scrap yards, they enjoy the chance to break up engines, or at least make their lives more difficult. Due to their grim task, they have earned a reputation on Sodor as "The Grim Messengers of Doom."
'Arry and Bert also have a tendency to act scheming, slimy, greasy, and oozy. According to their merchandise bios, they wish to scrap Oliver and Stepney, who have both escaped the cutter's torch once before.
However, despite their prejudices, the diesels have co-operated with steam engines on occasions, and when they are in trouble, steam engines often come to the rescue.
'Arry and Bert are based on BR Class 08 diesel-electric shunters. Over seventy members of this class survive today in preservation, while around 100 remain in service on the national rail network. They were in fact the most successful shunting engines in the world, with 996 of them built. Diesel, the Diesel Shunter, Splatter, Dodge, Paxton, Sidney, Ulli and the Mainland Diesels are also members of this class.
'Arry and Bert are painted green-grey with yellow hazard stripes on their fronts with yellow cabs. They have "Sodor Ironworks" written on their sides in white.