Choc-o-BoCo! is a magazine story. It was republished in 2005 in a simplified form as a story puzzle with new illustrations.
BoCo is helping the workmen to make a brand new tunnel in the hillside by taking away truckfuls of unwanted soil. But, one day, it starts to rain and the earth in BoCo's trucks turn to mud. The trucks did not like it either, bouncing about and sending mud flying all over BoCo. The other engines stare when BoCo returns to the Main Station and after being washed, he ends up as the last one back to the sheds.
During the night, it rains heavily and the next morning, while on his way to collect more discarded soil, BoCo runs into a landslide and is covered in thick, brown mud again. Luckily, BoCo isn't hurt, but he does need another hard scrubbing. Later, the Fat Controller sends BoCo to collect some goods from a factory. BoCo slows down to enter the factory siding, but he jumps right off a loose rail and smashes his way through the big, wooden factory doors. Inside, he hits an enormous tub of sticky, brown goo. It splashes all over BoCo's face as he grinds to a halt.
At last, a factory crane lifts BoCo back onto the track and he limps home. Just as a guard prepares to give BoCo a good scrubbing, BoCo stops him. BoCo sticks out his tongue and licks some of the goo off his face. The other engines gasp; thinking that the messy diesel had just eaten mud. BoCo laughs and tells them all it is actually chocolate!
- Gordon (cameo)
- Sir Topham Hatt (mentioned)
- This story's plot has some similarities to the sixth season episode, Percy's Chocolate Crunch and the magazine story Sweet Scent.
- Part of this story was rereleased in a 2003 Thomas and Friends magazine story puzzle under the name of BoCo's Bathtime!.
- When republished as a story puzzle in 2005, the exclamation mark was dropped from the title.
- In the 2005 version, the last two illustrations can be coloured in by the reader.
- The title for the story does not capitalise the "C" in BoCo. The title presumably relies on BoCo being pronounced incorrectly as "Boco".