Showing Off is a magazine story published in April 1997.
The Fat Controller puts BoCo and Diesel in charge of a shunting demonstration at the harbour for a party of schoolchildren. However, whilst being polished and cleaned, Diesel is very insistent that the shunting demonstration is his job, feeling supremely confident that he can do it without BoCo's help. But, BoCo ignores Diesel, knowing that he wants to show off his strength, and prove that he's the best diesel engine of all.
At the harbour, Diesel sets to work with the shunting demonstration, leaving BoCo feeling quite upset. When Thomas brings the schoolchildren to the harbour, they totally ignore BoCo altogether. BoCo tells Thomas that Diesel is showing off all his strength, and sure enough, Diesel begins to regret his boasting. But by then, it is all too late as all the weight of the trucks prove too much for Diesel, and he breaks down. Fortunately, BoCo is nearby and he carries on with the shunting demonstration, leaving Diesel feeling very silly; even Thomas can't help but tease him.