BoCo is a long green diesel with a rectangular face who works on Edward's branchline. BoCo has a brother, D5705, who is preserved at the East Lancashire Railway. His best friends are Duck, Bear and Edward.
BoCo arrived on Sodor in 1965 as a visitor. He once accidentally took Bill and Ben's china clay trucks in 1966. They decided to play a trick on him, playing on their identical looks to make the diesel think that one engine was disappearing and reappearing around him. Edward was nearby to sort out the confusion, and Bill and Ben soon came to respect BoCo.
Duck, who was suspicious of diesels due to his own experience, nevertheless took quickly to BoCo when the diesel took trucks to Tidmouth. The two started talking in the shed, but were interrupted when James arrived and called BoCo a "buzzbox". Ironically, James was attacked by a swarm of bees the next day, and so BoCo had to take the Express. Gordon was furious that a mere branch-line diesel got to pull a mainline train. Gordon was later stranded on Edward's branchline due to a signalman's error. Bill and Ben were quick to tease Gordon, and jokingly threatened to haul him away as scrap. When BoCo arrived with some trucks, he sent Bill and Ben to work, and the relieved Gordon felt he owed his life to BoCo. When Edward had trouble pulling an enthusiasts' train, BoCo helped Duck to shut the big engines up, who said Edward should be preserved. BoCo later filled in on the branch-line while Edward was at the Works. Donald and Douglas were the last engines to accept him.
In 1983, BoCo showed Old Stuck-Up around, but abandoned him when the new diesel insulted Sir Topham Hatt for keeping steam engines in service. Things had been smoothed over between James and BoCo by this time.
In 1995, BoCo also helped Edward to the Works when his bogie wheel snapped off, and ran the branch-line while Edward was undergoing an overhaul. He later met Victoria when Edward stopped at Wellsworth with her in 2007.
In 1991, Bill and Ben were once called to the yard to help shunt trains for BoCo. The two began fighting after an incident with the turntable and soon began to give each other the silent treatment. Edward thought this attitude was counterproductive, so BoCo came up with a plan: convincing The Fat Controller to let him take a special, BoCo's own heavy goods train was left behind, with only Bill and Ben to pull - neither would be strong enough to pull the train himself, so the two had to compromise to finish the job. Eventually in 1998, BoCo lost his buffer in keeping the twins in line and The Fat Controller decided to bring in a new diesel, Derek. Derek, however, had major teething troubles, and was prone to overheating. BoCo had to bring workmen to help Derek when he first broke down.
Although BoCo came across as bullying at first, he was still new to Sodor and turned out to be a good sort who was easygoing and good natured.
BoCo is based on a BR Class 28 Co-Bo diesel-electric engine, built at Bowesfield Works and designed Metropolitan Vickers in October 1958, approx. 56 ft 7 1/2 in long. His name is derived from his Co-Bo wheel arrangement, reversed in a way that Reverend W. Awdry thought sounded more affectionate.
BoCo was not immune to the mechanical failures of his real-life model, according to the Awdrys - however, the Fat Controller eventually found a fix to prevent this malfunction.
BoCo is painted in the green livery of British Railways in 1960s with yellow warning panels and has a grey face to match the other characters.
- Keith Wickham (UK/US; cancelled)
- Enrique Mederos (Mexico)