What was to ultimately be the final Railway Series book written by the Rev Awdry after the delay of a full year, focussed on Thomas's Branch Line, and introduced one last new character in the form of flighty young diesel, Mavis.  In spite of the fact Awdry thought he was scraping the bottom of the barrel with these stories, he still saw it through and wrote four more classic stories before finally deciding to call it a day with the books.  There was anticipation for a 27th, and Awdry had already chosen the title of "Really Useful Engines", but as we all know, he never got chance to use it...his son did however, but that's another story..!

Ghost Train
Percy tells Thomas and Toby about "The Ghost Train" that his Driver saw the night before, but Thomas is dismissive of the topic and calls Percy a "silly little engine".  Believing the story to be true, Percy tells his Driver that Thomas didn't believe the story, and the Driver laughs and tells him that it was a film he'd seen the night before.
That evening when Percy is returning from the Harbour, he runs into a cartload of lime that has been left lying on the track by a farmer.  When he reaches a nearby signalbox, the Signalman comments that he looks like a ghost, which Percy decides to use to his advantage to get back at Thomas!
Toby promises to help by alerting Thomas.  At first Thomas is confident when he hears about Percy's accident, and that his ghost is coming to warn them.  When the "ghost" puffs into the sheds, Thomas isn't quite as confident and scurries away in fear.
 
Woolly Bear
Percy is still milking the incident whereby he scared Thomas witless.  Thomas is still cross with him, and comments that he's a "Green Caterpillar with red stripes" and casts up Percy's constant lateness.  Percy knows it's not his fault as the gangers cutting the hay are delaying him, and tries to start home early on his next journey back from the Harbour.
But his plans are scuppered when he's hit by a boxload of treacle, and despite the best efforts of his crew, is still sticky when he leaves the Harbour.  There's worse to come when hay is thrown across the line by the wind and covers Percy completely, making him late again!
Thomas is cross about this, but when he sees Percy arrive, covered in hay, he soon lightens up!
Percy hopes to be cleaned before Toby arrives when he sees what he looks like in the mirror, but its no good!  Thomas tells Toby all about it, and the two engines spend the rest of the evening talking about Wooly Bear caterpillars and other creatures that crawl about in hay...Percy is unamused!
 
Mavis
Mavis the Quarry Diesel is causing trouble for Toby when he comes to collect his trucks.  Being full of her own ideas, she prefers to put the trucks where she wants.  Toby soon tires of it and leaves her to her own devices!
Mavis seeks Daisy's advice on the subject, and she backs Mavis up completely despite knowing nothing about trucks!
In light of bad weather, Toby tries to give Mavis the benefit of his advice before she ventures down a difficult part of the line.  Mavis takes no notice and does things in her own way, parking herself in the middle of the lane, and not using the gradient of the hill to help move her train.  Henceforth, Mavis finds herself stuck!
Toby is informed and refuses to help, but his Driver reminds him that the trucks Mavis is carrying are his really, and Toby remembers the trouble he could be in if the Fat Controller finds out!  He goes up and pushes Mavis back to a part of the line where it will be easier for her to move the trucks.  Embarrassed, she quietly leaves the trucks and hurries away home to her shed in the Quarry!
 
Toby's Tightrope

Mavis is reprimanded by the Manager, and reminded that she has no business going down Toby's line.  But Mavis thinks otherwise and contemplates plans to get out of the Quarry and down Toby's line.  She convinces her Driver to let them use a teeny bit of Toby's line and then persuades the trucks to push her down the line "accidently".
Unaware of the plans Mavis has, Toby decides to shunt the trucks himself.  As he does so, he falls into the truck's hands and they push him down the line, into further peril as Toby finds himself on a dangerous bridge above a raging river.
Mavis soon arrives and pulls the trucks and Toby to safety again.  He is most appreciative of Mavis's help, and so is the Fat Controller who honours Mavis's request to travel down the line from time to time.  Following the incident, Mavis becomes a more responsible engine and always listens to advice from more experienced engines.