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?

In 1886 the USA switched the rail gauge of the southern states to standard gauge. Most of the work was done over two days.

http://southern.railfan.net/ties/1966/66-8/gauge.html

Obviously doing this today would be a much more complicated affair, considering the much higher speeds and weights of contemporary trains.

If you want to, you can do it fairly fast. The decades plan is nonsense. It cannot take decades to change tracks, especially since the size of the rail network is quite small.

Toronto has 83 km of TTC-gauge streetcar track. 99% of it is set in concrete. This would take a little longer than 2 days.

If you want to nitpick about some corner case, go ahead.

I have this feeling that Finland's railway tracks are not set in concrete at street level ;)

But they are set into prefabricated concrete sleepers, which cannot be modified to a new track gauge and instead need to be swapped out completely. Whereas during the US gauge change, the railways used wooden sleepers, with the rails fastened simply with nails hammered into the sleepers, so it was simply a matter of pulling out the nails and hammering them in again at the new track gauge.