Finland announces migration of its rail network to international gauge

> will cost billions of euros, affect more than 9,200 km of track, and take decades

How is a change like this going to be implemented? E.g. are they going to mainly update some tracks everywhere (and have two systems running in parallel), or all tracks in selected areas (and have passengers change), or something else?

Was there a comparable large scale rail infrastructure change in some other country?

I think you can ”upgrade” the tracks to use three rails so you can handle both track widths. Also there are trains with adjustabe axle widths.

You’d need four rails, a 9 cm separation isn’t enough to fit two side by side. This solution has been ruled out as technically infeasible (I don’t even want to think about what the switches would look like…)

Adjustable-gauge rolling stock has also been ruled out as incompatible with the Finnish climate.

The most (only?) feasible way to do it is to “simply” build entirely new standard-gauge track next to existing track (and then possibly start upgrading the latter too at some point in the future).