For most of the nineteenth century U.S. railroads used a type of coupler known as a link-and-pin. This system required the switchman to guide the links into position and then drop a locking pin into the link. As a result of this dangerous job many switchmen lost fingers, limbs and even their lives. The Federal Safety Appliance Act of 1898 required the adoption of a type of coupler which would link on contact thus eliminating the need of a switchman's getting between the cars. In 1916 the Association of American Railroads adopted the Type D coupler designed by Major Eli Janney. Each coupler has a rotating knuckle. When the cars are pushed together the knuckles pivot closed and the pin drops into place automatically. A lever which extends from the coupler to the side of the car allows the pin to be lifted without requiring the switchman to get between the cars. The Type D evolved into the Type E which has become today's standard freight car coupler.
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The train couplers are said to be "buff" when the string of cars is pushed to the rear to maximize the slack between the knuckles of the couplers ( also known as "bunching the slack"). Pushing the slack to the rear in this way is the preferred (and sometimes the only) method of getting the locomotive to begin pulling the entire string of cars from a standing stop. In this "buff" configuration the locomotive starts out by pulling only the first car until the slack is taken up between the first and second car. Then the mass of the second car is added. Then the third etc. When the locomotive is finally pulling all of the cars along the track there is no longer any slack. In this case the couplers are said to be in "draft".