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).