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- “You have been busy bees!”
- ―BoCo to Bill and Ben
BoCo, retitled BoCo the Big Friendly Diesel in 2011, is the fifty-first book in the My Thomas Story Library series.
Bill and Ben find that their trucks are missing and realise a diesel has taken them. They go to investigate and find that a big diesel has indeed taken their trucks. They play a trick on the poor diesel to teach him a lesson. Edward calms the situation and Bill and Ben fetch more trucks. Bill, Ben, Edward, and BoCo the big diesel are now firm friends.
- This story is based on the story "The Diseasel" from the Railway Series book Main Line Engines.
- Railway Series illustrator Clive Spong is the co-illustrator for this book.
- On the 2008 cover, BoCo is depicted as a Co-Co and his British Railways logo is missing. Both errors are fixed on the 2011 cover.
- When Edward arrives, Bill is seen in between BoCo and Edward, but in the next illustration, Edward has moved right next to BoCo. Then Bill is back next to BoCo in the next illustration. Then Edward is back next to BoCo in the second-to-last illustration.
- Throughout the book, Bill and Ben's running boards have dark yellow basecoats instead of light grey.