Some people don't realise that steam locomotives, as well as other railway vehicles, have tyres. They are not pneumatic, as they are on cars, but are made of steel. The tyres wear with time, and it is much easier and cheaper to replace a tyre than a whole wheel.
It is important that the tyre doesn't slip on the wheel, or, even worse, come off. Our locomotive uses a method known as "double snip" to fix the tyre to the wheel.
The inside of the tyre is machined to be slightly smaller than the wheel which will go inside it. This is known as an "interference fit".
The tyre is held laterally by two shoulders (or "snips"). To fit the tyre, it is heated in a specially designed hearth, which causes it to expand. When it has expanded enough, the wheel can be carefully lowered into the tyre. As the tyre cools, it will contract and grip the wheel. The tyre is removed in a similar way.
When we had new tyres a few years ago, we had to have them made in South Africa. To have them fitted, we sent them to the South Devon Railway. They have a tyre heating ring that they acquired from the Great Western Railway workshops in Swindon. They kindly allowed us to take some photographs, and the video, to show the removal of the old tyres.