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That’s why trains have a trick called slack action. If you've ever been near a train as it starts moving, you probably heard a bunch of cracking that moves down the line of cars. The reason is that the connection from one car to the next is loose. So when the locomotive pulls the first car, the second car remains stationary until the slack is gone. With this trick, the locomotive can get one car moving at a time and add it to the group of moving cars. Pretty smart!