I’ve been wondering about this question for a long time. Every single car in a train has its own brakes, so the train should stop in the same distance that it takes one car to stop. Why is this not the case? Or does each car take a mile to stop on its own?
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The heavier something is, the greater the force that is necessary to change its speed. Trains are enormously heavy and so require enormous friction to change their speed even a little. A train slows down at a rate determined by the force of friction created by the brakes. The more friction, the faster the train slows. Since each car can contribute to this friction, the force is greater than when the brakes are applied in only one car. If one car locks its brakes, as has been pointed out, the wheels will slip because the friction is insufficient to stop the whole train. But when all the cars contribute, far more friction is achieved since the friction of all the wheels must be overcome before the entire train slips. In addition, the greatest friction is achieved between two surfaces just before one breaks free of the other. So the train will stop faster if the wheels don’t slip at all, just like a car stops faster if it doesn’t skid. Even then it will take a mile because of the immense weight of the train.
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