28) Here’s another picture taken inside a locomotive works, showing three men putting a tyre on a locomotive driving-wheel. Train wheels have tyres just the same as carts; they serve the same purpose, and they’re put on in the same way. You know that metal expands when it is heated. Well, when the tyre which is lying on the floor here, is heated by the squarish gas burners arranged round it, it will expand sufficiently to let the wheel slip inside it. And then, when the tyre cools, it grips the wheel tightly and the whole wheel is ready for the track. The tyre, of course, saves the wheel from getting worn, and can be renewed if necessary like the iron tyre of a farm cart.