Buff Load

A buff load is the compressive, or pushing, force seen within a train when cars are pressed together rather than pulled apart. It is the opposite of draft, which is the tensile force present when cars are being stretched out and pulled. Buff occurs whenever slack runs in, when a locomotive makes a shoving move, or when a train is slowing and the rear cars push forward against those ahead.

Buff Versus Draft

A train is rarely at rest in terms of internal forces. At any moment its cars are either being pulled apart (draft, tension) or pushed together (buff, compression), and the forces can shift from one to the other as the train accelerates, brakes, and moves over changing grades. Buff forces are carried through the couplers, draft gear, and underframes of the cars, with the draft system absorbing some of the shock as slack runs in.

Why It Matters

Excessive or abrupt buff forces are a concern for train handling and safety. A hard run-in of slack can damage cars or shift lading, and very high buff forces - especially on curves or with poorly distributed power - can contribute to a car being pushed off the rail, a hazard known as a buff-related derailment. Engineers manage throttle, braking, and slack to keep buff forces smooth, and equipment such as cushioned underframes and certain brake-control systems is designed to limit the severity of buff loads.