Finland announces migration of its rail network to international gauge

There ought to be good reason for optimism with this project. The land is already purchased so you “just” need to re-lay the track.

Ballast cleaners* are a thing and they are already pretty amazing at what they do, namely taking apart track and then putting it back, in place, from a machine that runs on those very tracks itself. I could imagine a giant version that not only cleans the ballast but also unties then reties the track back together at the new gauge.

* https://en.m.wikipedia.org/wiki/Ballast_cleaner

> The land is already purchased so you “just” need to re-lay the track.

While the details are unknown, this project will almost certainly mean new tracks alongside the old tracks at least for the main lines. Which means that the existing corridors in many places would not have enough space. Additionally there is probably desire to improve the geometry to allow higher speed trains, so that makes the existing corridors less useful

A ballast cleaner wouldn't be enough, because you basically need to swap out the sleepers, too, so you need a complete track-relaying train. And anything involving switches and crossings needs to be done conventionally, because those cannot be rebuilt by simply switching out the sleepers for standard gauge ones.

One such relaying train does 500m in one hour, including new sleepers. Seeing several such things in operation several times in several places, this should be doable without much modification when the gauge change is so small. Regarding switches and crossings, those come preassambled nowadays, and are just 'plugged in'.