It’s the same as the effort to reduce noise for transmission gears. Instead of having the teeth engage perpendicularly, they’re [angled](https://upload.wikimedia.org/wikipedia/commons/6/61/Gear-kegelzahnrad.svg). In the case of rails, the joint is [like this](https://i.ytimg.com/vi/Fc0GeJX5ltg/maxresdefault.jpg).
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