Figure 3.2. Freight Car Truck Assembly.
a. Truck bolster. The cross member in the center of a truck is the
truck bolster, which is designed to support the car's underframe and load.
The truck bolster is set crosswise of the truck and the matching car body
bolster rests on it, secured by the weight of the car and the matching of
the center plates on the bolsters. On freight cars, a loose center pin or
kingbolt acts as a guide to match the center plates during assembly when the
car body is lowered into position on the truck. If used on passenger cars,
the center pin connects the two bolsters so that the truck will not separate
from the car body. Study the relationship of the components in the inserted
sketch. The weight of the car's underframe and its load are transferred
through the bolsters, through a system of springs and bearings, and through
the axles and wheels to the rails.