BoCo is a mixed traffic diesel engine, who works on Edward's Branch Line.
BoCo once accidentally took Bill and Ben's china clay trucks. 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 and found out that he was good friends with Edward. 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 branch line 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.
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.
BoCo tried to cheer Gordon up when he got into trouble for spraying ashes onto a wedding party and later helped Gordon to Barrow-in-Furness when his firebars collapsed. BoCo also helped Edward to the Works when his bogie wheel snapped off, and ran the branchline while Edward was undergoing an overhaul. He met Victoria when Edward stopped at Wellsworth with her and later laughed at Donald after his accident with a set of buffers.
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 alone, so the two had to compromise and work together to finish the job.
When the first eight engines went to the mainland, BoCo was part of a group of engines designated to take over the major jobs of the railway.
Eventually, 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.