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).
Adjustable-gauge are used in Switzerland for a mountain line (Montreux-Interlaken) all through the year. I have never seen temperature issue mentioned.
https://www.gpx.swiss/en/stories/technology (the video is rather cool)
Nevertheless that’s what the initial feasibility studies showed. I don’t think the amount of complexity involved in that Swiss system would scale at all.
Given the small difference, maybe the easiest option is to "just" update the wheel axles of the entire fleet in the same time and at the same speed as the tracks.
Wheels are anyway wearing parts and are to be changed periodically.
Part of the motivation is removing the Russian gauge rails such that they can't be used in the case of invasion, so I don't think dual-gauge is really an option here.
