Coal and Treacle is a magazine story.
Diesel is in the mood for trickery. He spies a pile of stones and sprays oil all over them. Now, the stones look like black lumps of coal. Diesel then tells Percy that the Fat Controller wants him to use up the pile of "coal". Percy's fireman shovels the black rocks into the furnace and it is not long before Percy starts to feel unwell.
Harvey has to haul Percy back to the main station. There, Percy's fireman realises what Diesel has done. Percy is disgusted with Diesel, but the driver promises that they will get their own back.
The next day, Percy feels better and the Fat Controller sends him to the Sodor Sweet Factory with a load of treacle and he points to a wagon with "TREACLE" written on the side. Percy's driver gets an idea as he sees Diesel approaching and hastily removes the treacle signs from the wagon and drums. Percy's driver flags Diesel down and tells him that the filling station is closed and that he will have to fill up with the diesel oil. Just like the stones covered in oil had looked like coal, the treacle looks just like diesel oil. Diesel is happy to have his tank filled, but he does not get very far afterwards.