Stupidly enough the width of the railway track is not the same in all European countries. Notably, relevant for the article, it's different in Finland. But also in Ireland, Portugal and Spain.
The integration of the Baltic states into the European HSR system, Rail Baltica, including the Helsinki/Tallinn tunnel, are planned to be 1435mm.
https://www.dezeen.com/2019/11/21/zaha-hadid-architects-ulem... BTW. I'm definitely going to go there, by train, the year Rail Baltica is finished.
https://en.m.wikipedia.org/wiki/Variable_gauge
I once took a night train from Barcelona to Paris that changed gauge at the border. It slowed to a walking pace and travelled through a shed like structure that housed the equipment. There was some mild clunking and clanging from underneath as we slid over the mechanism. Quite impressive.