Slack

The motion, forward or back, that one or more cars, locomotives, or parts of a train has without moving other coupled cars, locomotives, or parts of the train. Loose slack is the free movement or lost motion between parts of a train. Spring slack is the movement beyond the free or lost motion brought about through compressing the draft gear springs. Slack is necessary so as to start one car at a time and so that the train may be operated around curves and over high and low places.