truck-bolster

from The Century Dictionary.

• noun A beam or cross-timber in the middle of a railway-truck, attached by a center-pin to the body-bolster, and supporting the car-body. See cut under car-truck.
• noun In a six-wheeled truck, a frame composed of two timbers at each end called spring-beams, resting upon springs, and one in the middle called a truck-center beam, the center-plate being secured to it, and the three timbers being connected by longitudinal iron bars or wooden beams.