Wheel arrangement The number of wheels per locomotive or car varies in both size and number to accommodate the needs of the railcar or locomotive. Regardless of these factors, pairs of identically sized wheels are always affixed to a straight axle as a singular unit, called a wheelset.

In technical railroad terminology, the wheels of a train are part of a larger assembly called a wheelset. A wheelset consists of two wheels rigidly fixed to a single axle, meaning they both rotate at the exactly same rate. The actual wheels are specially designed with a conical "tread" (the part that touches the rail) and a protruding "flange" on the inner edge that prevents the train from sliding off the tracks. These wheelsets are then mounted into a sub-frame called a bogie (in the UK/Commonwealth) or a truck (in North America). The bogie/truck contains the suspension, brakes, and sometimes the electric motors (on locomotives). The wheels themselves are typically "monobloc" (made from a single piece of forged steel) or, on older stock, feature a separate replaceable steel "tire" shrunk-fit onto a wheel center. This complex engineering ensures the train can "self-steer" through curves via the conical shape of the wheels without needing a steering wheel like a car.