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Ukraine is trying to get off the Russian standard and onto the rest of the world's standard (4 feet 8 and 1/2 inches - the extra half inch was added to give the rails some play). Russia has a different standard, as I understand it, to make it harder for the Germans to invade Russia.

The extra 1/2-inch play was needed because trains tended to stick on curves; George (and/or Robert) Stephenson found this out the hard way. My favourite rail factoid is that Brunel's broad gauge was 7ft 1/2in, with the 1/2in added for exactly the same reason.