Mighty Magnet! is a magazine story.
Rosie is told to deliver some scrap parts to the scrapyard and feels really useful. Gordon says that she is only taking useless junk, making Rosie cross. Rosie later rolls over a damaged rail that bounces her load out of her truck and onto Gordon's express line. The scrapyard inspector quickly uses his crane magnet to grab the metal blocking the line just as Gordon thunders through.
This story appears to be a retelling of the 2011 magazine story Rosie's Really Useful Scrap!