- "You're just as old as Edward was when they first started calling him 'old'."
- ―BoCo's Driver, Samson, BoCo, and the Man in the Hills
BoCo is a wise, old, friendly green diesel who works at Brendam and on Edward's Branch Line.
BoCo was given a stressful welcome from Bill and Ben with their antics, but eventually became well at home on the North Western Railway. He works happily with Edward, and the 2 engines are very good friends.
BoCo and Mavis braved an incredible windstorm to rescue Scruff from a collapsed shed at the end of the Little Western Extension. On their return journey, the bridge over the Arlesdale Railway had split in 2 and BoCo quickly came up with a plan to save his friends. He pushed Scruff to safety, but caused Mavis to derail on the second attempt. When the bridge collapsed, BoCo was stranded on the other side of the valley. Luckily, both turned out to be alive and well at the end of the calamity.
During the Munitions Incident, BoCo and Edward defied General Zen's orders several months later to rescue Derek after he had broken down. When his friends were trapped, BoCo charged at full speed into the inferno so Edward and Derek could travel to safety. The fire brigade quickly found BoCo and put out the flames.
Later, BoCo had to take a load of quarry stone over to the Crater Lake Bridge, which was undergoing repairs. However, the responsibility of telling BoCo was left to Bill and Ben, who decided to play a joke on BoCo by telling him to take it to Kellsthorpe Refinery. As a result, BoCo bypassed his real location, but stopped there for a brief few moment to talk with Den, wherein BoCo found about the cliff-diving competition at the lake. After stopping at the refinery and having to listen to Walter Sliggs, BoCo put 2 and 2 together, realizing that the twins had tricked him, and that he should have known that since his driver had to argue with the signalman. BoCo raced back down to the Crater Lake bridge, but the trucks took advantage of his hurry to send him down the gap in the bridge!
BoCo was later rescued, and whilst resting at the works, was told by The Fat Controller that his accident at Crater Lake was considered spectacular enough by the judges of the cliff-diving competition to be awarded third-place, and he was given a bronze trophy for his "Swan Dive" off the lake!
One day BoCo was sent to collect a train of hopper cars from the Crovan's Gate Mine. After ribbing Samson regarding his many blunders, BoCo departs. Not far into his journey, BoCo overheats and is unable to continue, prompting the mine foreman to send Samson to retrieve his train. As Samson arrives, an explosion in the cliffs causes a rock slide, BoCo frantically orders Samson to push both of them to safety. Before they can escape, a pile of fallen rock obstructs their escape, causing BoCo to brake suddenly. The sparks from his brake's ignited some dynamite, which in turn breaks up the blockade and allows BoCo and Samson to continue on. BoCo thinks they're home free, until an unforeseen rock slide causes BoCo to crash and derail. After their ordeal, BoCo thanks Samson for saving him.
After a brief stay at the Crovan's Gate Works, BoCo returns to service. Meeting Arthur on one of his trips, they discuss the shady circumstances surrounding the explosion. Arthur suggests talking to Edward about the situation, but BoCo refutes this idea, knowing Edward has probably already heard of the situation, and in fear it may further distress his friend Edward.
While filling up with fuel, Philip met him following his relocation, annoying the big diesel with his chattering.
BoCo is friendly, hardworking, and always puts others before himself. He is a bit of an over-active martyr, and tends to put himself in danger even when it does not seem entirely necessary. BoCo wants everyone to be safe and takes a great satisfaction from feeling valued and trusted. He is a loyal friend and kind to all engines, diesel or steam. He may lose patience with Bill and Ben from time to time, but he deeply values their companionship.
- Season 0: Old Groaner (does not speak)
- Season 1: The Old Warrior (cameo), Snow Blind (cameo), Scruff's Scaffolding (cameo)
- Season 2: Two Hearts Burn Together (cameo), Scot-free (cameo), Mavis and the Tornado, Rosie (cameo), Rock-Star, Pummeling Percy (cameo), Tag-Team (cameo), Munitions, Swan Dive, Swashbuckler (cameo), Thomas and the Fortune Teller (cameo)
- Season 3: Marion Saves the Day (cameo), This Too Shall Pass (does not speak), Samson, BoCo, and the Man in the Hills, Culdee Fell (does not speak), The Suggestion Box (cameo), Penn Pals (cameo), Percy and Asbestos (cameo), Stafford's Request (does not speak)
- Web Clips: Mike and Frank Banter: SLOTLT, ScarletFire, and EE93's Return (cameo), Gina gives Percy some pointers - Meet the Characters! (cameo), Percy's Kafka Dream (cameo)