Double Teething Troubles is the twelfth episode ever made in Season 5.
Bill and Ben argue over who has rights to some trucks nearby, and, despite BoCo's warnings, derail each other at the points. The Fat Controller decides to send a Diesel to help them, but the Diesel, Derek, is new and has problems with his cooling system, as seen after he breaks down on Gordon's Hill. The news soon spreads, and Percy, who misunderstands Thomas when he mentions Derek has teething troubles, tells the horrified twins the Diesel has a toothache. The next day, Derek arrives and helps the twins with a long train of China clay trucks. However, Derek's engine overheats, and he stops. Bill and Ben pull him and the trucks and Derek is sent to the works. Bill and Ben apologize to BoCo for being rude and they and Duck talk about teething troubles all night long.
- It is said that Bill and Ben heard an unfamiliar whistle (Derek's), but Diesels have horns.