The steel wheels of railway cars are fitted with tires which are themselves usually made of steel. The tire is a hoop of steel that is fitted around the steel or iron wheel. No obvious form of fastening is generally used to attach it. As with wagon wheels, the tire is held by an interference fit - it is made slightly smaller than the wheel on which it is supposed to fit. To fit a tire, it is heated up until it is glowing hot. Railroad workshops generally have special equipment to do so. As the tire heats, it expands, making it big enough to fit around the wheel. After placing it on the wheel, the tire is cooled, and it shrink fits onto the wheel. When cold, friction between the tire and the wheel is such that the tire will not budge even under quite extreme forces.