How Freight Car Trucks Work
How Freight Car Trucks Work
A freight car truck is the wheeled assembly that supports the car body and rolls along the rail. Most North American freight cars ride on two trucks: one near each end of the car. (Specialty cars built for exceptionally heavy or oversized loads, such as Schnabel cars, can ride on many more than two.) The car body rests on these pivot points, much the way a tabletop rests on its supports. This arrangement lets a long, rigid car body negotiate curves, switches, and uneven track, because each truck can swing and adjust independently of the body above it.
Why a Car Has Two Trucks
Placing a truck near each end distributes the weight of the car and its lading across multiple wheelsets and a wider span of rail. It also allows the car to curve. As the train enters a curve, each truck swivels about its pivot so that the wheels can follow the rail while the car body stays comparatively straight. Without this swiveling action, a long wheelbase would bind against the rails in anything but a straight line.
The Three-Piece Truck
The dominant design in North American freight service is the three-piece truck. The "three pieces" that give it its name are the major castings:
- Two side frames (also written side frames), one on each side, which form the structural sides of the truck.
- One bolster, a transverse beam that spans between the two side frames and carries the car body.
Everything else - the spring groups, the wheelsets, the brake rigging, and the rest - is a separate component that works with these three castings rather than being counted among them. The two side frames and the bolster are not bolted into a rigid frame. Instead, the bolster floats on the spring groups, and the whole assembly is held together largely by the loads passing through it. This loosely connected, self-aligning design is part of why the three-piece truck has remained the standard for so long: it is simple, robust, easy to repair in the field, and essential to letting the truck flex and steer as it negotiates curves.
The Main Components
- Wheelsets and axles: each wheelset is a pair of wheels pressed onto a common axle, with the bearings press-fit onto the axle ends - the bearings are an integral part of the wheelset. A truck carries two wheelsets.
- Bearings: press-fit onto each axle end, the bearings let the axle rotate freely while carrying the truck above them. Modern freight equipment uses sealed roller bearings.
- Side frames: the cast steel sides of the truck. They rest on the wheelsets through a roller bearing adapter that sits on each bearing, and they house the spring groups. The load path runs downward from the body, through the bolster and springs, into the side frames, through the adapters and bearings, and finally to the wheels on the rail - so the wheelsets carry the side frames, not the other way around.
- Bolster: the cross member that transfers the car body's weight down into the springs, and through which the car body pivots.
- Spring groups: clusters of coil springs that cushion the bolster and absorb track shock. Some spring groups also include hydraulic snubbers to add damping.
- Friction castings (also called friction wedges or friction dampers): wedge-shaped castings, energized by springs, that press against the side frame columns to damp the vertical motion of the bolster. They convert unwanted bouncing into friction so the car rides more steadily.
How the Truck Swivels
The car body sits on the bolster through a center plate and bowl, with the body's weight bearing down on the center of the truck. This connection acts as a pivot, letting the truck rotate relative to the body as the train moves through curves. Side bearings near the ends of the bolster limit how far the body can roll from side to side.
A Note on Standards
Freight truck castings, wheelsets, and bearings are not designed in isolation. Because cars routinely move between many different railroads, key components are built to common industry specifications so that parts and repairs are interchangeable nationwide. The AAR publishes standards that govern truck castings and related components, helping ensure that a side frame or bolster made by one supplier will fit and perform consistently across the fleet. This shared standardization is what allows a single freight car to roll safely over the lines of dozens of railroads during its service life.