Main Line Engines is the twenty-first book of The Railway Series published in 1966. The book contains four stories:
- The Diseasel: Bill and Ben are tank engine twins that work at the China Clay Workings on Edward's Branch Line. One day they leave some trucks in a siding and leave for more, but return to find them all gone. Their drivers determine it was a "diseasel" that took them, and when Bill recalls a sign saying "coughs and sneezles spread diseasels", in his words, Ben blames him. Their drivers quieten them and formulate a plan. They find the diseasel on a siding, and manage to confuse him so as to make him giddy, but when Edward comes along he reprimands them and tells them to apologize to the diseasel, who calls himself BoCo. Edward sends them away, and he and BoCo joke about the twins.
- Buzz, Buzz: Duck and BoCo are talking about Bill and Ben. Duck tells BoCo he has nicknamed them "The Bees", when James, who just arrived, claims he would simply blow smoke at bees to shoo them away. The next day, two porters are pulling a trolley with a beehive on top when it falls off and bees swarm out. They decide to rest on James' boiler, but when one burns its feet, it stings James on the nose, and James runs away. In the end, BoCo takes the Express, and that night the others talk about a red beehive on wheels.
- Wrong Road: Although the big engines are forbidden to go on the branch lines because of their weight, Gordon claims he is forbidden because they are "vulgar", and leaves to take his late train. He misses his "path" after a lady with a large floppy green hat confounds the fireman into thinking the guard had waved his flag, and in the end Edward's train leaves ahead of Gordon. However, this creates a huge disruption as the Wellsworth signalman isn't informed, and Gordon is left on a siding at Brendam. The next day, Bill and Ben come into the yard, and tease him by "mistaking" him for scrap and arguing about how to dispose of Gordon. Gordon thinks they're serious, and when BoCo arrives, he asks for help. BoCo sends the twins away with his trucks, and Gordon has had great respect for diesels ever since.
- Edward's Exploit: Edward is set to take some enthusiasts home, but finds it hard to move the train. After meeting up with Bill and Ben, Edward takes the enthusiasts away, but stops when a crank-pin breaks, battering his splashers and frame. His crew take both crank-pins off, and Edward manages to struggle home. Edward is sent to Crovan's Gate, and BoCo does his work so well that he single-handedly changes Donald and Douglas' opinion about diesels!
The book contains examples of:
- Artifact Title: Despite the name of the book, the majority of the book takes place on Edward's Branch Line.
- Baby Carriage: The bees escape from their hive in "Buzz, Buzz" thanks to a trolley stopping suddenly to avoid running over an old woman.
- Big "SHUT UP!": An irate Duck said this to Henry, Gordon and James who found Edward's ordeal hilarious.
- Black Comedy: Bill and Ben continue arguing, in increasingly graphic detail, about whether to have a mortified Gordon cut up into scrap or hurtled into the sea in "Wrong Road".
- Body Horror: In "Edward's Exploit," not only does Edward's crank pin break, it causes his left side rod to spiral out of control and bend his splasher and frame out of shape! His agonized expression says it all.
- Bratty Half Pints: Bill and Ben. Edward lampshades this by stating they're "maddening at times", despite that they don't mean any harm.
- Dark Is Not Evil: BoCo. It did seem that a diesel would stir up trouble much like Diesel in previous episodes, but BoCo had mistakenly taken the trucks and didn't know it was Bill and Ben's.
- Determinator: Edward in the last story.
- Fantastic Racism: Some of the engines display their low opinion of diesels. At first...
- In "Buzz, Buzz", Duck disliked the presence of BoCo until he found out that he knew Edward and the two became friends.
- In "Wrong Road", Gordon frowns upon diesels until BoCo comes to stop Bill and Ben's teasing him; he thinks the diesel saved his life.
- In "Edward's Exploit", Donald and Douglas are the last of the engines to accept BoCo.
- The Freelance Shame Squad: The other engines laugh at James' red nose.
- Identical Twin ID Tag: Bill and Ben's nameplates, which the drivers remove as part of a plan to get their china clay trucks back from the diesel.
- It Belongs in a Museum: Gordon scoffs that Edward should give up and be preserved before it's too late.
- Jerks With Hearts of Gold: Gordon and James in the end of "Edward's Exploit". They teased Edward for struggling with the coaches, but became respectfully silent after hearing about his ordeal.
- "Just Joking" Justification: The narration mentions in the end of "Wrong Road" that Bill and Ben were only teasing Gordon.
- New Meat: Gordon's fireman is mentioned as being new on the job and winds up confusing a woman's green hat with the guard's flag, causing him to start the train early and leave half the passengers and the guard behind.
- Non-Indicative Name: Despite the book's name "Main Line Engines", most of the book takes place on Edward's branch line.
- Put on a Bus: Due to injuries sustained from his breakdown, Edward is sent to the Works and does not appear again for the remainder of the Rev. W. Awdry's tenure as author of The Railway Series. He eventually returns in Christopher Awdry's run.
- Scary Stinging Swarm: "Buzz, Buzz".
- Screw This, I'm Outta Here: Upon being stung by a bee, James runs away, deserting the express and leaving it to BoCo to pull.
- Sibling Rivalry: Bill and Ben shows shades of this when they argue about who is responsible for spreading the "diseasel" with their "coughs and sneezles".
- Team Dad: Edward to Bill and Ben. Duck mentions to BoCo how he's the only engine to keep the two in order.
- Trickster Twins: Bill and Ben in "The Diseasel".
- Very Loosely Based on a True Story:
- "Buzz, Buzz" is based on a real event in which six bees escaped from a hive at London Bridge Station.
- "Wrong Road" is based on a real event that occurred at Sittingbourne, Kent in 1911.
- "Edward's Exploit" is based on a real event of what happened to the crosshead assembly of an express train near Settle, North Yorkshire in 1960.
- What Happened to the Mouse?: We know what happens to Gordon in "Wrong Road", but not what happened to the passengers of Edward and Gordon's trains, both of which ended up in the wrong destination. Likewise, even if Henry was among the engines teasing Edward, we don't know if he had an accident after taking Edward's passengers on another train.
- What the Hell, Hero?:
- In the end of "The Diseasel", Edward scolds Bill and Ben for accusing BoCo of stealing their trucks.
- In "Wrong Road", BoCo scolds the twins for teasing Gordon, threatening to take away their trucks if they don't behave.
- At the start of "Edward's Exploit", Edward also scolds the twins for teasing Gordon, and then told Gordon it served him right, which made him cross.
- Whole Episode Flashback: The events of this book take place in the latter half of 1965.
