Spain uses high speed trains on standard gauge next to slower trains on Iberian gauge, which is also wide. That would be a good way to do it for Finland. Built the Talinn-Helsiniki tunnel and then a standard gauge high speed rail line from Helsinki-Tampere-Oulu as a bases and then it becomes much easier to just change gauge on the slower lines.
Spain uses high speed trains on standard gauge next to slower trains on Iberian gauge, which is also wide. That would be a good way to do it for Finland. Built the Talinn-Helsiniki tunnel and then a standard gauge high speed rail line from Helsinki-Tampere-Oulu as a bases and then it becomes much easier to just change gauge on the slower lines.