Taking inspiration from the history of the Ravenglass and Eskdale Railway in Cumbria, Awdry introduced a new set of characters to expand the Sodor Railway network and set the premise for Duck to have his own branch line later on.  Like the Mountain Engines, these characters were to be peripheral compared with the others and appear rather infrequently in future.

Ballast
When the Fat Controller begins replacing ballast on the railway, Donald and Douglas often disappear down a line that the others never used to collect it.  They come back saying that “vera wee engines” bring trucks down from the hills.  The others don’t really believe the twins, but Duck wants to see for himself so he asks to take some trucks.
When he arrives at the yard, he sees what the twins mean when he meets a small engine called Rex, and later his two friends, Bert and Mike.  They tell him they were bought from another railway in England, that has closed, to work on the Island of Sodor.  They’ve been brought over to collect ballast from the old mines and take passengers in the holiday season.  Duck, quite taken with the three newcomers, leaves and promises to bring them passengers for the upcoming holiday season.
 
Tit For Tat
Whilst being prepared for work one morning, Bert is informed that he’ll be taking two special visitors on a tour of the railway, one who takes moving pictures and another, who writes books – the Fat and Thin Clergymen.  When they first meet, Bert is quite impressed with the visitors and his Driver offers them a ride in his cab, but they decline in favour of following the train by road to take photos instead.  Bert’s view of the visitors sours when they don’t wave at him when he passes them by.
Things get worse when the Clergymen’s car splashes Bert with water as they rush past in the ‘Lane’.  Bert is cross and decides that he will pay them out for it.  He gets his chance when the Fat Clergyman accepts a ride in Bert’s tender on the return trip.
Bert makes use of a hill on the way back with branches, which create a sort of natural tunnel over the track.  Because it has been damp the night before, Bert knows exactly what will happen when his Driver has to use full steam to get him over the top.  The steam from his funnel shakes the leaves so hard that they shake the water off and cause it to pour all over the Fat Clergyman and the passengers.  At first, Bert thinks it a great joke, but soon realises how silly he’s been when the Small Controller sends him to the shed to think about what he’s done.  Fortunately, the Fat and Thin Clergymen are forgiving and set to work cleaning Bert and promise to write about him and his friends in a new book.
 
Mike's Whistle
After Duck’s Driver and Fireman cook over his whistle the night before, some of the egg they were preparing has become caught in his whistle valve, and causing it to make a strange noise.  Mike thinks it great fun to mimick Duck’s whistle, and make rude remarks about the sound it’s making.  Rex and Bert laugh and point out that Mike’s whistle isn’t that great either, something he takes great objection to.  To prove them wrong, he sets out to take his train and proudly shows off his.  But on the journey, there’s trouble.  At the Top Station, the Driver hears a rattle and makes a note to tighten it at the next stop.
But when a stray cow steps onto the line, Mike tries to “shoo” her away by whistling, which causes his whistle cap to shoot off completely and land in an adjacent field by the side of the line.  To no avail, and protests from the passengers, the crew return unsuccessful.  Mike is anxious – there are boards and places he needs to whistle at.  But the passengers have a great idea – they’ll whistle for him!
Mike returns to the yard to find there are no spare whistles available, and he’ll have to do without for a while.  At the end of the day, Rex and Bert tease him for being so silly.
 
Useful Railway

Toward the end of the running season, the local farmers use the small Railway to transport their sheep’s wool to market.  Rex is told by the Small Controller that he is to take the 1st train of wool and feels the responsibility deeply.  After a safe load-up, and a good initial run, Rex is confident, but he doesn’t count on Willie the Farm Worker.  
Running late with his load, he hurries off on his tractor without his trailer full of wool being tied properly.  As he speeds into the yard, the load becomes upset all over the line.  Willie tries to warn Rex of the danger ahead, but it’s too late and he derails onto his side.  The crew clear up the mess, but leave Rex by the side of the line, where he’s given pitying comments by passengers and cheeky looks by Bert and Mike as they pass.
At the end of the day, he’s helped home by Bert and Mike and welcomed back to the yard with good news – the wool traffic has been a success, the passengers have been impressed by how well the railway coped in an emergency and they’ve been promised more of both in the new year.  All three small engines are delighted and proud.