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In most parts of the world, railroad tracks have a gauge of 1435 mm, or where I come from, 4 ft 8 1/2 inches. This is the standard gauge, first adopted in Britain and now used by 55% of the world's track mileage. But that leaves 45% not using this standard
And as you can see in the dark purple on this map, a large chunk of that 45% is Russia and the many countries that were once ruled by it. They use a 1520 millimeter gauge, or 5 feet, appropriately called "Russian Gauge".
The lack of a standard gauge meant that the Russian Empire was isolated from Europe's rails and was a barrier to trade.
On the plus side, it meant that any invaders could not use their trains on Russia's tracks. Although that advantage is null since retreating troops can just tear apart the rails as they withdraw.
So what happens if Russia goes for standard gauge?