Posted speed limits will be very conservative, rather than a theoretical maximum. In the real world the track curvature will have minor variations, the cross-level (superelevation) will vary somewhat, rail and wheel profile will have an influence, as well as the truck behaviour. At least center of gravity is fairly predictable for passenger trains. Freshly rehabilitated track is quite likely to have some imperfections through settlement. While trains and freight cars running away on steep downgrades have successfully negotiated some sharp curves at surprisingly high speeds, that just proves there is a comfortable factor of safety used.
On the highways we see advisory speeds for curves, which for the most part are also very conservative under good driving conditions. Of course speeds grossly in excess of that suggested will similarly have bad results.
John
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