Sir Topham Hatt, nicknamed "the Fat Controller" (previously known as "the Fat Director"), is the head of the North Western Railway.
The Fat Controller has had an obsession with locomotives since he was a child, and running a railway of his own was his dream to come true.
The current Fat Controller started out as "The Fat Director" and became the Fat Controller in succession of his father. During his time as director, he dealt with more minor issues, such as Henry getting stuck in the tunnel after refusing to get his paint spoiled in the rain. Taking rather harsh precautions, The Fat Director closed the big green engine in the tunnel where he was never to be released from it. However, when Gordon broke down with the Express near the tunnel, the Fat Director found it only fitting to release Henry and have him redeem himself and save Sodor's Premier Express.
Soon afterwards, the Fat Director became the Fat Controller when the North Western Railway was nationalised. This was also when he received his baronetcy as "Sir Topham Hatt" due to his contributions to the railway.
When the Fat Controller was on holiday, he was visiting the old Sodor Tramways line and met an old tram engine named Toby. His wife, Lady Hatt, and children, Stephen and Bridget, took a ride in his coach Henrietta and they all befriended the old steam tram. Unfortunately, soon after the Fat Controller left, there was trouble back on Sodor! Thomas had "broken the law" for riding on public rails without cow catchers and side plates on his branch line. The Fat Controller thought this was outrageous, but then he remembered the old steam tram from his visit to the closing tramway, and wrote a letter there to have Toby come and stay on the Thomas' Branch Line and occasionally help at Ffarquhar Quarry with the new engine Mavis, all thanks to the kind-hearted controller.
The Fat Controller is called upon if there are issues such as troublesome engines, derailed trains and whatnot on the North Western Railway.
The Fat Controller is firm but fair. He can be strict, but also shows a fatherly side to the engines. He is quite a strong character and will remind the engines that he is in charge when they start to rebel against his authority. He does not stand bullying and even though he is usually serious, he does enjoy a good laugh. Despite his kindness and fairness, however, there are times where he berates the engines for situations beyond their control.
The Fat Controller is a short, stout gentleman. He is usually dressed in a black suit with grey trousers, a cream or yellow waistcoat, a black tie and a top hat.
- Season 1 - Geoffrey's Real, Percy's Oil Spill, Splatter and Dodge, Edward Tries His Best, Henry the Warrior, The Great Western Way (mentioned), Harvey's Goods Train, Scottish Engines, Skarloey Comes Home, Salty's Secret Story, Oliver and the Slip Coaches, Snowdrift, Big Strong Murdoch, Waterworks, Bill and Ben, Scruff the Scrapper, Bert's Milk Train (mentioned), Fergus and the Small Engines, Gordon, Spencer and the Flying Scotsman (does not speak), and Tinsel on the Tracks
- Season 2 - Disappearing Diesel, Unique Engines (mentioned), Haunted Henry's Forest, The New Controller, Jealous James, The Return of Boulder (mentioned), Duncan's Discovery (mentioned), The "Ghost" Warrior, Back to Misty Island, Sidney Gets Lost, Sodor Day, The Ghostly Tale of Timothy (does not speak), Timothy and the Aquarium, Trouble with Brake Vans (mentioned), and Ministry
- According to Elizabeth, the Fat Controller was a bad driver in his younger days. When he is seen driving Winston, it seems his skills have not improved.
- Throughout the television series, the Fat Controller is accompanied by two men in standard blue railway uniforms. They are presumed to be his bodyguards.