We only have three days to travel by train from Geneva to Zurich, stopping for two nights in the Berner Oberland region. If I thought the weather would be clear, I'd be more likely to stay in a town higher up in the mountains and also to spring for the more expensive tourist trains with scenic windows. Is June a rainy month or more clear? Does it tend to rain all day when it does rain? I'd prefer to play it by ear and make travel and lodging decisions based on the weather forecast once I get there, but it seems like reservations need to be made far in advance for the more desirable routes and accommodations.
The VIP seats on the Golden Pass trains from Montreux to Zweisimmen are out of service for the foreseeable future, so don't worry about paying extra on that train. Going from Interlaken to Zurich via Luzern, all the cars are panoramic.
when you say that you have 2 nights in the Berner Oberland that means that you only have one full day there. That is courageous.
Most days it will not rain all day - but it does happen. I always have at least 3 full days there, 4 nights, so that if the weather is not great on two days I still have one up my sleeve.
The mountains make their own weather. On a clear day in the morning is the best time for going up. Often the cloud will start to bubble up late in the morning or early afternoon and by late morning the summits are shrouded in fog or cloud and visibility is poor. There will be plenty of sun lower down. There are often thunderstorms around in the evening.
But every day is different. To put all your bets on one day is brave.
I'd prefer to play it by ear and make travel and lodging decisions
based on the weather forecast once I get there, but it seems like
reservations need to be made far in advance for the more desirable
routes and accommodations.
Playing it by ear is definitely the way to go in Switzerland when you've got your eye on the mountains! Train reservations far in advance are not needed at all as Swiss trains almost never sell out. Even if one train sells out you can catch the next one 30-60 minutes later. And there are always plenty of good lodging options to be had at the last minute.
The mountains are the place to be and I'd say it's worth staying up in Muerren or Gimmelwald in order to be there when the clear weather arrives. When I once stayed five nights in Denali National Park, the only clear view I had of the mountain was at 5:00 a.m. on about day three. Believe me - I jumped out of bed to see it.
If for some reason it's unseasonably miserable, it's always possible daytrip another direction by train.