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I'm not sure why you think rails are safer than rubber tires. Metal-on-metal contact has way less friction, which makes for much worse stopping distance and worse safety especially at crossings. Having grade-separated dedicated infrastructure _does_ improve safety, especially if there's no human drivers involved, but we can do that just fine with pavement in tunnels or elevated roadways.

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Rails are safer because vehciles are more predictable - they are always on the track, with only limited places where they can switch tracks, and you can control that externally ensuring there are no conflicts. Rails can handle more people because in the form of a train they can pack in a lot more people. Cars use a lot of space for engine and luggage compartments.

Very good points... intuitively, rail seems safer (and simpler to automate?), but your points have changed my mind... Now I wonder though, how do modern roller-coasters stop so suddenly? Magnetic breaks?

Electromagnetic brakes are used, but the biggest difference is that the weight of passengers is a much bigger portion of total weight (you don’t have a locomotive and the cars are not nearly as heavy per passenger) a decelerating quickly is part of the appeal. If passengers on trains were unencumbered with stuff and strapped in as on roller coasters, they could be built to do it too.

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