The Twin Engines is the fifteenth book of The Railway Series published in 1960. This contains four books:
- "Hullo Twins!": The engines are getting dragged down by the amount of work they have, and they are pleased to hear a goods engine is arriving from Scotland. However, the Fat Controller is surprised to hear that two engines have arrived and goes to meet the engines, but only manages to discover they have "lost their numbers", as they put it, and they have since named themselves Donald and Douglas. The Fat Controller orders for them to have numbers painted on them and puts them to work, but is determined to find out which engine should not be there and send him back.
- The Missing Coach: Donald and Douglas (now #9 and #10 respectively) realize they only have numbers painted on their tenders. One afternoon, Douglas is shunting in the Yard and worries about being sent away, unfortunately forgetting to shunt Thomas' Special Coach. The twins decide to swap tenders so as to convince the Fat Controller that Donald has taken his goods train, but he isn't fooled and reprimands Douglas severely.
- Break Van: The Fat Controller threatens to send one of the twins away and they behave, until a spiteful brakevan delays Douglas' goods train. Donald soon puts the van right, but when Donald crashes into a signalbox and James has to do his goods work, the brakevan and the trucks, sensing James' fury, hold back, and Douglas has to help as a "banker". The brakevan holds back again, but gets its comeuppance after Douglas accidentally crushes it. The Fat Controller comes to supervise the clean-up, and has mixed feelings about Douglas staying.
- The Deputation: Winter comes early, and Donald and Douglas ask to help clear the line. They manage to rescue Henry from a snowdrift, but they are still worried that they will be sent back. Edward tells Percy that they need a "deputation", and the big engines elect Percy to be their "disputation". Percy tells the Fat Controller that Donald and Douglas will be scrapped if they return to Scotland, but the Fat Controller has already made up his mind: he gives the twins a new coat of paint and allows them to stay.
The book contains examples of:
- Armor-Piercing Question: "Douglas, why are you masquerading with Donald's tender?"
- Asshole Victim: The Spiteful Brakevan. James even mentions how glad he is that it is broken to pieces and never repaired.
- Beat:Henry: What is a desperation?
Percy: It's when engines tell The Fat Controller what's wrong.
Duck: Did you say "tell The Fat Controller"?
Gordon: I propose that Percy be our...disputation.
- Berserk Button:
Duck: Tinpot indeed! Let me tell you!
- For Duck, calling the Great Western Railway "tinpot railway".
- For the twins, having their whistles being made fun of, as Gordon and Henry found out.
- For James, being reminded of crashing into the tar wagons.
- Big Brother Instinct: Twin-brother instinct, actually (Though, it's confirmed Donald is the older one, so it still counts). Donald was furious when the Spiteful Brake Van caused Douglas' trains to be late and gave him a big bump.
- Some of the dialogue also implies that Donald has helped to smuggle Douglas over to Sodor to save his life.
- Big "SHUT UP!": James says this to Gordon and Henry who laugh when Donald reminds him of the tar accident.
- Bully Hunter: The twins stood no nonsense from the troublesome trucks and stood up to Gordon and Henry for making fun of their whistles. Donald was this when he gave the Spiteful Brakevan a hard bump on behalf of Douglas.
- Call-Back: Douglas' recall of James crashing into tar wagons in "Dirty Objects".
- Chronic Villainy: Even after one hard bump from Donald, the Spiteful Brake Van returns to his troublesome ways as he tells the trucks to hold James back.
- Don't Split Us Up: Donald and Douglas really didn't want to part from each other. This even provides the quote and picture of the tropes page.
- Earn Your Happy Ending: Despite their troubles with the Special Coach, the signalbox, and the Brake Van, Donald and Douglas are here to stay in The Fat Controller's railway. Cue all of the engines whistling with joy.
- Felony Misdemeanor: Gordon complained about how the twins were going to be sent away because Donald accidentally broken a signal box followed by James with breaking the Spiteful Brake Van.
- The Freelance Shame Squad: Gordon and Henry laugh about Douglas' remarks about James' accident with the tar wagons.
- Got Volunteered: Percy in "The Deputation".
- Jerkass: The Spiteful Brakevan.
- Karmic Death: The Brake Van was unintentionally broken by Douglas. This is fitting considering that he was the one that caused his trains to be late.
- Oh, Crap!: Percy after being volunteered to confront the Fat Controller. Also, he almost jumps when he comes before him.
- Skewed Priorities: The Signalman got annoyed with Donald for jamming his points, rather than endangering his life.
- Smug Snake: The Spiteful Brake Van as learned by this quote:Spiteful Brake Van: You can't. I'm essential.
- True Companions: In "The Deputation", all of the engines are quick to defend the twins despite the trouble they caused previously.
- Twin Switch: Donald and Douglas do this after the latter forgets to shunt Thomas' Special Coach, but they are soon found out by The Fat Controller.