Small Railway Engines is the twenty-second book of the Railway Series.
Some leadmines up in the hills have long been closed, but their waste-heaps still spoil a lovely valley.
The Fat Controller has now found that this waste is good weed-killing railway ballast. He talked to the Owner and the Thin Controller of the Skarloey Railway, and other important people. They "went shares" and built a Small Railway to fetch it away.
New ballast is being laid in the trackbed. Donald and Douglas bring it from a line that starts at Tidmouth, and all they will say is "verra wee engines" bring it down from the hills. Duck is curious and asks to take some trucks along the line. There, he meets a small green engine called Rex, who tells him about the "chute" that loads his trucks. The next time Duck visits, Rex introduces him to two other engines, Bert and Mike. When Duck discovers that they don't have any passengers yet, he leaves, promising to bring some for them.
Bert's driver tells the engines that two clergymen are coming to take pictures of them and put them in a book. Bert thinks he will be physically stuck in a book, and is apprehensive, but cheers up when he meets them. However, he gets cross again when they don't wave at him while taking pictures, and the final straw is when they accidentally drench him with muddy water when they pass him in their car. Bert plans revenge, and at a hilly part he creates steam, soaking his driver and the Fat Clergyman, who was riding in the cab. But later, after both parties make up, Bert tells the others they will be in the book along with him.
Duck's whistle is faulty after his crew used his fire to cook eggs, and Mike makes fun of him, saying "engines without whistles aren't engines at all". The others tease Mike about his whistle, and when he blows off steam he is chosen to take the passenger train. Mike is furious and is in a bad temper all day. When a cow strays onto the line, Mike tries to 'shoo' her away by whistling, but all that happens is that his whistle shoots off into the sky. Everyone tries looking for it, but to no avail, and they have to make the whistling noises for him. Mike is sent to work at the quarry for the rest of the day, and to top it off Bert and Rex tease him again in the shed.
After an encounter with a flock of sheep, Mike calls them silly and can't understand why Rex calls them useful. The Small Controller arrives and tells them they are now arranging to take wool trains for the local farmers. Rex is chosen to take the first train. Rex is over-confident, but gets his comeuppance when a tractor's load spills onto the line, derailing him. Bert and Mike tease him, but say sorry when they come to take him home, and the Small Controller tells them their railway is more popular than ever.
- Donald or Douglas
- The Small Controller
- The Thin Clergyman
- The Fat Clergyman
- Gordon (not seen)
- Willie's Tractor (does not speak)
- The Fat Controller (does not speak)
- The Ballast Spreader (cameo)
- Henry (mentioned)
- James (mentioned)
- "Mike's Whistle" may have been loosely adapted into the sixth season episode, Faulty Whistles. Duncan and Peter Sam replace Mike and Duck respectively. The actual story itself will be adapted in the twentieth season, along with "Tit for Tat" and "Useful Railway", making all three stories the first Railway Series stories since the fourth season's Mind that Bike to be adapted into television, while "Ballast" would be the only story to not be adapted due to the Arlesdale engines already being introduced in Sodor's Legend of the Lost Treasure.
- The Reverend acknowledged the help given by members of the Ravenglass and Eskdale Railway Preservation Society in the preparation of this book.
- At the end of Useful Railway, there is a short message about the Ravenglass and Eskdale Railway: "If you have enjoyed these stories, you will also enjoy a trip to the Ravenglass and Eskdale Railway in Cumberland".