Uranium235
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Yes, Yashur is right.
Calculation:
The Earth's radius is approximately 6371 km, the length of the railway from Omsk to Moscow is about 2676 km, and the distance between the upper and lower bunks is about a meter.
Let's assume the railway is a perfect arc (and we won't be too far off).
2676/6371 * (1/6371 - 1/6371.001) = 42 centimeters.
Those who are not lazy can calculate with all the errors, and in general, they say that the distance between the bunks is not a meter, but 923 mm.