The wheels of a locomotive are mounted on a chassis known as a truck and are designed to keep the engine aligned with the tracks. Each wheel tapers slightly from the inside outward, which helps to steer the train around curves, and is fitted with a projecting rim or “flange” on the inside edge. Ordinarily this should not touch the track, and is a safety feature to prevent the train from derailing. Wheel sets—two wheels joined by an axle—are variously sized and perform subtly different functions: large driving wheels are powered by the pistons of the locomotive, while smaller, unpowered leading and trailing wheel sets support the weight of the engine and enable the train to pass through junctions and bends in the line.
Each leading and trailing wheel set is mounted on a frame beneath the car. The strength and rigidity of the structure, or “truck,” enables the wheel sets to resist torsional forces when the train turns. Most trucks fix wheel sets in place in an inflexible frame, but “steerable” trucks allow the axles to rotate laterally around a pivot between the two wheel sets, increasing the stability of the train around bends. Modern trucks also house the train’s braking and suspension systems.
How it works
The flanged wheel was invented in 1789 by English engineer William Jessop. The raised rim on the inner wheel edge prevents derailment and does not touch the rail during normal running, unless the track is poorly maintained. The conical edges of the train wheels allow the wheel sets to slide across the heads (tops) of the rails, enabling the train to follow curves. Engineers observed that the characteristic side-to-side swaying action of a train in motion was due to its tapered wheel sets wobbling up and down the railheads in order to “hunt” for equilibrium. They termed this movement “hunting oscillation.”