draft

Draft is the longitudinal tension force transmitted through a train's coupler system when cars are being pulled apart or stretched. During operation, slack naturally develops between coupled cars as acceleration, braking, and grade changes occur. When that slack is taken up, the draft gear (the mechanical cushioning device inside or behind the coupler) absorbs and transmits the pulling force. This tension is what keeps the train coherent as a unit rather than allowing cars to separate.

The magnitude of draft depends on train weight, locomotive pulling power, and track grade. A freight train on level track might experience draft forces in the range of several hundred pounds per car during normal operation, but mountain grades or emergency braking can push this much higher. The draft gear is engineered to handle these forces through springs and friction elements that compress and extend as the train moves. Older cars used friction draft gears with bronze or cast iron friction wedges; modern equipment typically uses hydraulic or spring-friction hybrids for more consistent performance across temperature ranges.

The coupling cycle between draft and slack is fundamental to rail dynamics. When a locomotive accelerates from a stop, couplers move forward but cars lag slightly, creating slack. The locomotive continues pulling until all the slack is taken up and draft begins; at that moment a shock load transmits through the train. Conversely, when the locomotive slows or brakes, cars tend to catch up and pile against one another. A skilled locomotive operator manages throttle and brake to minimize these shock loads, reducing wear on couplers, draft gear, and car structures.

Excessive draft indicates serious problems. If draft forces exceed the design rating of the coupler or draft gear, the assembly can fracture or the car can derail. Worn draft gear loses its cushioning ability, so full shock loads transfer directly to the coupler knuckle, shortening its service life drastically. Inspectors check draft gear operation by observing how freely the coupler moves and by noting any permanent deformation, leaking fluid, or broken springs when the train is stationary.

The term 'draft' itself comes from the verb 'to draft', meaning to pull or draw. In rail terminology it refers specifically to this tensile condition, distinct from 'buff' forces (compression from cars pushing together). Both draft and buff stresses are managed by the coupling and draft gear system as fundamental aspects of keeping a train mechanically intact during the entire operating cycle.