Too Fast! is a magazine story.
One day, Diesel is in a very bad mood; the Fat Controller has given him Thomas' work to do while the little tank engine is being serviced. Diesel sets off along Thomas' Branch Line with some workmen who are to cut down the long grass beside the track. It is very slow work and Diesel hates it.
Soon, Percy puffs past on the other line. Percy cannot resist the opportunity to call Diesel a slowcoach which annoys Diesel even more; even Percy is going faster than him. Eventually, Diesel's trucks are full and he makes his way towards the main station. The grass will be stored in the yard there overnight and Thomas will deliver it to the hill farms as winter feed the next day. Diesel rattles along a full speed, causing his trucks to rattle and bump. They tell him to slow down, but stubborn Diesel doesn't listen.
Meanwhile, Percy is already back at the main station yard. The Fat Controller is showing him a new timetable, but suddenly, the timetable is blown out of his hand by the wind. The piece of paper flies away, beyond the station. Percy's driver jumps on his bicycle and gives chase to it. The driver catches up with the timetable when it gets caught in a bush. He picks up the timetable and begins to cycle back to the main station.
Suddenly, Diesel comes clattering around the bend, much too fast. One of the trucks bounces right off the rails and spills its load. Just then, Percy's driver came cycling round the bend. His brakes have failed and he cannot stop. Percy's driver then spots the pile of spilled grass and rolls towards it. The driver hits the mound of grass and topples off his bike. The Fat Controller rushes over to see if he is all right. The driver is fine, but is very relieved that the pile of grass cutting is there; it was the only way he knew he could stop safely.
The Fat Controller then speaks to Diesel. He says that he is always warning the stubborn diesel engine about his speed, but this time, he has actually managed to prevent a nasty accident. Diesel is pleased that the Fat Controller is not cross with him, but he doesn't like having to wait for the truck to put back on the rails and reloaded. Diesel is determined that he won't have another derailed truck; it takes far too long.