Tolerances
The railways have a fault tolerance regarding the real gauge. Finland allows their railways to be between 1514–1554 mm (less tolerance for higher speed).
This means that, on the condition that the gauge of the rolling stock is kept with certain limits, through running between 1,520 mm (4 ft 11 5⁄6 in) railways and Finnish 1,524 mm (5 ft) railways is allowed. Since both 1520 and 1524 are well within tolerances, the difference can be said to be mostly a paper difference.
The new Sm6 high-speed train is specified for a 1522 mm gauge. High-speed trains have less tolerances against gauge error, but this way through running works well.
Read more about this topic: Russian Gauge