truck

In railway equipment, a truck is the wheeled undercarriage assembly that supports a rail car and allows it to move along the track. It consists of a steel frame mounted on two axles, each carrying a pair of wheels that roll on the rails. The truck pivots at its center, allowing the car body above it to rotate slightly as the train goes around curves. Without trucks, a rail car would simply sit on the rails; the truck is what actually moves it.

Most freight and passenger cars use two trucks, one at each end, connected to the car body through a kingpin that allows rotation. Lightweight cars or short wheelbase vehicles might use a single truck. The truck frame itself is built from rolled steel sections, typically I-beams or channel iron, welded together to handle the static load of the car and its contents, plus the dynamic stresses of acceleration, braking, and cornering. Axle boxes or journal bearings sit at the ends of each axle, and the wheels are either pressed onto the axle or keyed in place.

Trucks vary significantly by load class and intended service. Freight trucks carrying heavy loads use heavier frames and larger wheel diameters, typically 33 to 36 inches for standard gauge, compared to 24 to 28 inches for lighter passenger car trucks. Bolster trucks, used on some flatcars and depressed-center cars, have a much simpler frame since they carry load directly on the bolster. Radial trucks, fitted with side bearings that allow greater lateral movement, became common on freight equipment in the mid-20th century because they reduce wear on flanges when traversing tight curves and switches.

Problems in truck maintenance center on wheel wear, bearing overheating, and frame fatigue. Wheels develop flat spots from sliding brakes or seized bearings, forcing periodic truing or replacement. Hot boxes, the overheating of journal bearings, can cause bearing seizing and derailment if not caught early; modern cars use roller bearings and thermocouples to prevent this. Truck bolts loosen under vibration, and side bearing wear increases truck hunting, the lateral oscillation that degrades ride quality and damages track.

The term "truck" in this context comes from the low platform or frame used in 18th-century horse-drawn rail wagons. It has remained standard vocabulary in North America; British railways use the term "bogie" for the same component. Understanding your car's truck type and maintenance condition is essential because truck problems cascade into derailment risk, track damage costs, and unplanned repairs that halt revenue service.