Redefinitions
In the late 1960s the gauge was redefined to 1520 mm in the Soviet Union. At the same time the tolerances were lessened to increase speed.
In Finland Finnish State Railways kept the original definition of 1524 mm, even though they also have lessened the tolerances in a similar way.
Estonia after independence redefined its track gauge to 1524 mm to match Finland's. The redefinitions did not mean that a lot of actual railways or rolling stock were changed. It was more a rule change regarding new and renovated tracks. See Track gauge in Estonia.
Read more about this topic: Russian Gauge