Tradman, I found another nice video (though nothing new with respect to railway curving):
Tradman
Watch the first one minute, and then repeat what you said, without blushing:
120m of rail cannot curve 1.12mm?


The rail bows because of the weight of the ties free-floating in the air, but that does not mean it wasn't level when affixed to the earth. Further, Tradman stated that they
collectively cannot curve around a ball; this is because the point where each of those segments meet would not be flush after a certain distance and the rail would no longer be level.
It's the same issue that Edward Hendrie covered in his book The Greatest Lie on Earth when it comes to constructing skyscrapers. Each one of the steel beams must be machined to be flush with the others so that the structure is square, otherwise, the skyscraper will bow outward and be unstable. The slight variances in measurement, like the 1.12mm you so mockingly cite, can be catastrophic from an architectural standpoint over both horizontal and vertical distances.