<P> The use of tires is becoming obsolete . The utilisation of traditional freight wagons was often so low that tires never needed renewal, so it was cheaper to fit a one - piece ("monoblock") wheel . Monoblock wheels are lighter and offer better integrity as there is no tire to come loose . Modern flow - line repair lines are disrupted by the inspection of the wheel centre once the tire is removed, possibly generating extra rectification work, and the need to make each tire fit its allocated wheel centre . Monoblock wheels are now more economical . </P> <P> The most usual cause of damage is drag braking on severe gradients . Because the brake blocks apply directly on the tire, it is heated up, relaxing the interference fit . It is not feasible to fit the tire with such a heavy interference as to eliminate this risk entirely, and the retaining ring will ensure that the tire can only rotate on the wheel center, maintaining its alignment . In rare instances the rotation could be so severe as to wear the retaining ring down till it breaks, which could result in derailment . </P> <P> Severe braking or low adhesion may stop the rotation of the wheels while the vehicle is still moving can cause a flat spot on the tire and localized heat damage to the tire material . </P> <P> Tires are reasonably thick, about 3 inches (76 mm), giving plenty of room for wear . Worn tires or tires with flats are re-profiled on a wheel lathe if there is sufficient thickness of material remaining . </P>

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