A truck consists of side frames, wheels, axles, journals and journal boxes, a bolster, springs,
bearings, and brake rigging. Besides connecting the wheels to the locomotive, the truck must carry
vertical weight, absorb lateral thrusts, support the tilting tendency of the locomotive, travel on uneven
track without distortion, carry the brake rigging, and, on some locomotives, carry the traction motors. A
pair of guides, called pedestals, is supported by the side frame and holds the journal boxes. Some
locomotives have chains attached to the underframe and to the truck to limit the swing of the truck in
case of derailment.
The swivel truck, shown in figure 1.9, is used on all but the smallest diesel-electric locomotives.
Each swivel truck has a center plate which supports the locomotive frame superstructure and allows the
truck to turn freely on curves. Tilting of the superstructure is limited by side bearings mounted between
the underframe and the truck frame. The principal types of swivel trucks in locomotives larger than 40
tons are four-wheel rigid and four- and six-wheel swing bolster.
Figure 1.9. Swivel Truck.
In a rigid truck, the bolster and frame are cast together; in the swing bolster truck, the bolster is
independently suspended and can absorb some of the shock between the truck and body. For this reason,
the swing bolster truck is more suited to heavy road service. Because six-wheel swing bolster trucks