Roller coasters are designed to not exceed a certain speed at the top of a loop or hill. This is done to minimize the chances of cars derailing. Therefore the acceleration of a train is carefully calculated for any given roller coaster depending both on the loop as well as on the weight and length of the train. Under certain weather conditions however, for example during strong headwinds, the acceleration might not be enough to push the train through the highest point of the loop or over a hill.
Now imagine - or try it at home - if you hang something, for example a sock, over a rod it only stays there, if both ends are equally heavy. If you clip something to one end, the sock will slide in that direction unless you adjust it by dangling more of it on the other side of the rod. The force that's pulling on both ends of the sock is called gravity. The same force also affects the train. If you have headwinds, only the first 1-2 cars of the train might get pushed through the highest point of the loop. If the train is 6 cars long and all cars are equally heavy, that doesn't put enough weight on the other side of the loop and the train rolls back. But if they load the heaviest people in the front, the first two cars might be so much heavier than the other four that under the same circumstances two cars can pull the whole train through the loop.
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