The Railway Series
BoCo once took Bill and Ben's trucks of china clay by accident, so the twins decided to play a trick on him. With their nameplates and numbers removed, they crept into the yard and ran up alongside him one at a time, making him believe that there was only one annoying and very swift tank engine. Edward came along and sorted the situation out, and Bill and Ben both apologized to BoCo, who gained their respect.
BoCo later took a train to Tidmouth, then went to rest in the shed. Duck, who was suspicious of diesels due to his own experience with one, took quickly to BoCo, and befriended him as they were talking about the twins. Their conversation was rudely interrupted by James, who told Duck not to be afraid of bees and called BoCo a "buzzbox". Ironically, James was attacked by bees the next day, so BoCo took the express for him when he fled the station.
Gordon was infuriated that a branch line diesel had taken such an important main line train, and complained to Edward about this. Due to an error on behalf of the signalman, Gordon was diverted onto Edward's Branch Line and had to stay the night at Brendam. The next morning, Bill and Ben arrived at the harbour and teased Gordon, threatening to scrap him. When BoCo arrived with a train of trucks, he sent Bill and Ben away to work. Gordon, who did not know the twins were joking, believed that BoCo had saved his life and gained a newfound respect for him.
When Edward was struggling to pull out of the big station with an enthusiasts' train, BoCo helped Duck defend him when the bigger engines were teasing him. After Edward was sent to the Works, the Fat Controller put BoCo in charge of filling in for him on the branch line. Donald and Douglas were the last to accept him. (RWS; Main Line Engines)
After Gordon accidentally blew soot onto a group of wedding guests at Wellsworth, BoCo tried to cheer him up. He later helped Gordon take the express to Barrow-in-Furness after his firebars collapsed. (RWS; Gordon the High-Speed Engine)
When important goods arrived at Wellsworth yard, Bill and Ben were both sent to shunt the trains for BoCo and the other engines. However, after the two ran into each-other by mistake, they began arguing with each-other and complained to BoCo about each-other behind their backs. In order to have the twins get along again, BoCo devised a plan, which he suggested to the Fat Controller; he would leave his heavy goods train for the twins to take to the harbour, which would be too heavy for only one twin to pull. Both twins reconciled and took the train to the harbour. (TVS; One Good Turn)
BoCo found himself struggling to keep the twins in order, so the Fat Controller arranged for a new diesel engine, Derek, to come to the clay pits and help them. Unfortunately, Derek suffered from teething troubles, and overheated as he was making his way up Gordon's Hill. BoCo came up the hill, and helped Derek out as he was being mended. (TVS; Double Teething Troubles)
BoCo is based on the British Rail Class 28 Co-Bo diesel-electric engines, which were the only engines in the United Kingdom with a Co-Bo wheel configuration. These engines were problematic, and suffered from many mechanical issues. Only one member of the Class 28s has been preserved, D5705, and is currently on the East Lancashire Railway. BoCo's number, D5702, originally belonged to a real Class 28, which was scrapped in 1969.
In the Railway Series, BoCo is painted BR green, his bufferbeams are painted red, and his cab roof is grey. He has two faces on both of his cabs, which both differ from each-other; the lower part of his first face is yellow and the top half is green, while his second face is fully yellow. He carries both his number and the British Railway logo on his sides.
In the television series, BoCo is painted light green, his cab roof is painted white, and his bufferbeams are painted red. He carries his number and the British Railway logo on his sides. He also has a grey face, so as to match the other characters.