The Fat Controller
This book focuses on Thomas's branch line. Percy plays a trick on Thomas, but later runs into trouble himself. Meanwhile, The Fat Controller has hired Mavis, a diesel engine working for the Ffarquhar Quarry Company, to help out while Thomas is absent, but Mavis is very headstrong and thinks Toby is an old fusspot. She pays no attention to his advice and causes a great deal of trouble, but eventually comes to Toby's rescue when his heavy load pushes him across a crumbling bridge.