The steel wheels of railway cars are usually fitted with steel tires because
the rolling friction of steel is much less than that of rubber.
(Some trains, mostly certain types of metros & people movers, have rubber
tires, including some lines of the Paris Métro, the Mexico City Metro, the
Caracas Metro, the Montreal Metro, & the Santiago Metro).
Although steel may seem resistant to damage, wear still takes place - on
acceleration, on braking, & on cornering. As well as the simple wearing away
of the wheel surface, a wheel that wears begins to deviate from the correct
profile. The shape of a train wheel is designed & specified precisely for the
best possible riding & cornering characteristics, & too much wear can alter
that. Wear can also take place unevenly if wheels lock up under heavy braking,
causing flat spots.
Another different form of damage to a train's wheels takes place if violent
wheelslip occurs. The friction so caused can heat the wheel (& rail) enough to
cause permanent heat damage.
Replacing a whole wheel because of a worn contact surface proves expensive, so
train wheels are fitted with replacable steel tires. 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, & it shrink fits onto the wheel. When cold,
friction between the tire & the wheel is such that the tire will not budge
even under quite extreme forces.
Removing a tire is done in reverse - the tire is heated while on the wheel until
it loosens.
Tires are reasonably thick, up to about an inch thick or more, giving plenty of
room to wear. If a tire wears out of shape, or gets flat-spotted, but has a
reasonable amount of metal left, it can be turned on a wheel lathe to refinish
it, reshaping it to the correct profile.
