[eqvinox]

The really annoying thing is that it's too close for "simple" dual gauge rails (e.g. 1435 + 1000); 1435 + 1524 is possible and in fact exists (e.g. the one single SE-FI railway bridge that exists is dual guage: https://openrailwaymap.org/?style=gauge&lat=65.8273204537081...), but AFAIK it's expensive because the mounts interfere and need to be quite custom.

[ta1243]

That bridge is 4-rail, not 3-rail

https://en.wikipedia.org/wiki/Torne_River_Railway_Bridge#/me...

Even if you were to 4-rail every line, you'd potentially run into loading gauge issues (you would have to offset the current centre of the bogies, go too far one way and you collide with platforms, too far the other way and you collide with oncoming trains)

[eqvinox]

Bleh, but kinda confirms my point too. I do think there are some 3-rail setups in other border regions though? I should check… then again it doesn't matter that much if it's 3 or 4.

As for the loading gauge, yes, of course. On the plus side, this is Finland, most of the lines is in the middle of nowhere and single track even. Maybe the best option for them is to just build 1435 in parallel whereever possible, and just merge where not otherwise practical (bridges, tunnels, populated areas & stations). I don't even think it's that infeasible considering Finland's layout. I'd wager there are only a handful of specific locations that need expensive work.