Drawing inspiration from several narrow gauge Railways in Wales and the events of one in South America, Awdry wrote about Duke.  Basing the character on Prince of the Ffestiniog Railway and the Mid Sodor Railway itself primarily on the Corris Railway, Awdry wrote a heart-warming and touching book which delved into the past of Sir Handel (Falcon) and Peter Sam (Stuart), and explained about the Grand-Father figure locomotive they left behind on their old Railway, and how he was repatriated once more!

Granpuff
The story opens introducing us to three engines - Falcon, Stuart and Duke, or as he's more affectionately known - Granpuff!  The three engines work on their own line in the mountains of Sodor and live a contented and quiet life.
Duke is a stickler for discipline and doesn't like the younger engines stepping out of line, "That would never suit his Grace!" and takes the opportunity to remind them about No.2.  He was a bad character who rode roughly and flounted the rules, he was soon put in his place, permenantly in fact, and was put to work as a pumping engine behind the sheds!
But as the years draw on, the line goes into decline and the Railway is forced to close down and sell the engines.  Falcon and Stuart are sold, but Duke is too old to be of any use.  Instead, he's placed in his shed out of the way, and left there for several years.  Whilst Stuart and Falcon move on and gain new names at their new home, becoming Sir Handel and Peter Sam respectively, they never forget their old mentor.
 
Bulldog
Peter Sam raises awareness of Duke when he tells the other engines that at Skarloey and Rheneas's 100th Birthday party, the real Duke never came.  Despite protest from Duncan that the Duke was real, Skarloey still urges Peter Sam to divulge more information.
He tells them the story of what happened when Duke saved Falcon.  They had been due to take the Mountain Road as a double-header, Duke had offered to lead, but Falcon refused the offer, prefering to head the train himself to see the view ahead.  Duke is quick to remind him to be careful, and does so throughout the journey, but Falcon takes no notice.  As a result of his foolishness, he ends up coming off the rails and ending up hanging procariously from a cliff!
Duke holds tight to the young engine and keeps him from going over the edge, but it's clear that the old engine will need more than willpower to rescue Falcon, hence the need for more water is urgent.  Water is taken in every manner from a nearby Platelayer's cottage until Duke's thirst is quenched and he is able to pull Falcon back onto the rails again!
The young engine thanks Duke for his bravery and Duke receives acclimation from the passengers too.  But the hero is modest, and merely says, "Ah well, you've just had a new coat of paint, it would have been a pity if you'd have rolled down the mountain and spoilt it!"
 
You Can't Win
Duke's Picnic train is a special that runs in the summer months for visitors to the railway, he is always intent upon pulling the train, even when he's unwell.  One morning, Duke felt unwell after taking "The Picnic" and returns to the shed to have his tubes cleaned out, only to be harranged by Stuart about Duke getting old and that he and Falcon will have to take better care of him in future!  Duke is indignant and starts away to collect his train.
But Duke suffers trouble as he's bringing his return train home, and has to be helped back home by Stuart and Falcon, a task which Stuart muses in!  At the station, the cavalcade split up and Stuart pulls Duke home behind the passenger train.  Still loving the situation, Stuart is unaware that Duke still has enough power to continue, and is saving a little surprise for him later on!
Duke forces the train along and makes such a noise with his effort, people come to see what's happening!  At the Works Station, a boy asks his father why there were two engines, and the father mistakenly suggests that Stuart needed help!  Stuart is furious and vanishes in a cloud of steam, whilst Duke reminds him, "You can't win!"
 
Sleeping Beauty

A band of explorers, including the Fat and Thin Clergymen, get together in order to find Duke again.  The Small Controller offers his services to them and duly allows the men to travel along his line in order to find Duke.
The men spend days scouring the hills trying to find Duke, but each time find nothing but disappointment.  That is until however, the Fat Clergyman unexpectedly falls through the roof of Duke's old shed and lands on top of him!
Duke is startled to see them, but when the group explain about Falcon and Stuart, and finally fetch "His Grace" the old engine is very happy again.
Early the following morning, Duke is finally retrieved from the old shed and taken to his new home on the Skarloey Railway.  And even though they wake him up, Duke is pleased to see his two old friends, and that little has changed about their cheeky characters!  But most of all, he's happy to be free again and being given the chance to be useful again!