Train wheel

The wheel acts as a rolling component, typically press fitted onto an axle and mounted directly on a railway carriage or locomotive, or indirectly on a bogie (in the UK), also called a truck (in North America).

Regardless of these factors, pairs of identically sized wheels are always affixed to a straight axle as a singular unit, called a wheelset.

The purpose of the separate tire is to provide a replaceable wearing element – an important factor for steam locomotives with their costly spoked construction.

In modern times the tire is invariably made from steel, which is stronger than the cast iron of earlier eras.

The brake shoes (or blocks) are applied directly to the wheel surface which generates immense amounts of thermal energy.

Alternatively, severe braking or low adhesion may stop the rotation of the wheels while the vehicle is still moving, which may cause a flat spot on the wheel-rail interface and localized heat damage.

[citation needed] Modern railway wheels are manufactured reasonably thick to provide an allowance of wear material.