BoCo is a mixed traffic diesel engine, who works on Edward's Branch Line.
BoCo was built for BR in 1958, and along with the rest of his class worked in the Barrow-in-Furness area from 1962. In 1965 he found his way to the Brendam Branch Line. Sir Topham Hatt expressed interest in buying him, and BR allowed him to keep the engine for trials. These proved satisfactory, and BoCo was purchased.
BoCo once accidentally took Bill and Ben's china clay trucks. The twins 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 signalmen'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 the Fat Controller 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 branch line while Edward was undergoing an overhaul. He met Victoria when Edward stopped at Wellsworth with her while being brought back to service on Thomas' Branch Line 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.
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.
BoCo is a tame diesel engine, who is sensible, kind, and has a decent sense of humor. Upon his arrival, he was unfortunate enough to be set upon by the mischievous Bill and Ben, when, thinking he had intentionally stolen their trucks, they pretended to be one engine and nearly made his eyes pop out. BoCo is now wise to the tank engine twins' pranks though he still somewhat humors them to a point. He is also good friends with Bill, Ben, Edward, and Duck. He is a true friend and is often willing to stick up for the underdog. BoCo is firm, but fair and friendly in his nature, not being afraid to speak his mind. Even Gordon soon thought of him in high regards when he "saved" him from Bill and Ben's antics. BoCo upholds a more praiseworthy and respectable, likeable image for diesel engines everywhere, being happy to show the more pompous steam engines that diesels have their uses. BoCo helps bring good order across Edward's branch line, even occasionally pulling main line trains, as the Fat Controller recognises how versatile he is.
BoCo is based on a BR Class 28 Co-Bo diesel-electric engine. His name is derived from his Co-Bo wheel arrangement, though reversed as the Reverend W. Awdry thought "BoCo" sounded more affectionate. These engines were initially used on the "Condor" express freight services but the class's Crossley engines quickly proved problematic. They suffered frequent failures, and in 1961 the entire class was handed back to the manufacturer for remedial work on the engines, and to cure problems with cab windows falling out while running. Upon return to service in 1962, all members of the class were allocated to Barrow-in-Furness. BoCo was susceptible to the mechanical failures of his class - according to the Awdrys, however, the Fat Controller eventually found a fix to prevent these malfunctions. The real engines were so unfortunate, and most were withdrawn and scrapped by 1969.
BoCo's number is that of a real Class 28. The real D5702 was built in October 1958 and was withdrawn in September 1968, before being scrapped at J. Cashmore Scrapyards the following year.
In the annuals, BoCo mentions a brother of his - D5705, a real preserved locomotive, who is the last surviving Class 28. In another annual, BoCo claimed to have a twin called "CoBo" in order to play a trick on Bill and Ben.
BoCo is painted in the Brunswick green livery of British Railways with yellow warning panels. In the television series, he is painted in a lighter shade of green without yellow warning panels. The number "D5702" is painted on his cab sides in white.
In the Railway Series, BoCo has a yellow face. However, in Main Line Engines, he was first depicted with a yellow end from below his mouth to half-way up his nose. The top half of BoCo's nose and his eyes were in his green paintwork. This is evident in the fourth and sixth illustrations of "The Diseasel" as well as the last illustration of "Edward's Exploit." This type of face was also shown in "Old Stuck-Up" in James and the Diesel Engines. Since he had two faces at each end of him, this could explain why his face differs in different illustrations. In the television series, he has a grey face on one end only to match the other characters.
- Unlike most of the characters within the Railway Series, BoCo's number was real. The real engine carrying this number was withdrawn in 1968 and scrapped in 1969.
- BoCo's name is often mis-capitalised as "Boco".