Mountain Mistake is a magazine story.
Early one morning, the Fat Controller comes to see Oliver and instructs him to take some trucks up the big mountain as there has been a rockfall that has blocked the line. Oliver steams away and it is not long before the men working on the track are loading rocks into Oliver's trucks. Oliver is delighted; the job has been finished quickly and the Fat Controller will be impressed. The trucks overhear him and are not happy; they think Oliver should try carrying big rocks like they do, so they put on their brakes. Oliver keeps pulling and soon hears a creaking sort of noise. The truck behind Oliver had split right down the middle. The Fat Controller is not annoyed and says it is about time someone taught the trucks a lesson.
Now, whenever Oliver has to work with the trucks, they are as helpful as can be. They think Oliver is the strongest engine in the world.