Nutshell Booking Dates

I do not have booking numbers for this but in general September is near season (not top season) and what you mentioned with an often changing weather mix.
My guess is that spontaneous (weather depending) trip booking shall be possible at planned date of travel. Maybe worth an e-mail to NiN provider?

The train tickets you can book also on https://www.vy.no/.

Weather forecast: https://www.yr.no/en

I'd guess it might matter whether there's a large cruise ship sitting in Flam on the day you ultimately choose.

I suspect the riskiest segment to book late is the Flamsbana, but I know nothing about ridership on the mainline train between Myrdal and Voss--a segment most Nutshell travelers skip but which it appears you will need to take.

Your trip is starting from Flam.

You thus have the luxury of being able to start off before the day tripping Norway in a Nutshell people have arrived from Oslo and after cruise excursions have left. You also don't have to take the visitor cruises, but the local ferry run by Lustraabatane.

That is the 11.00 boat from Flam (or the 0830 from Gudvangen). Same route, same scenery.

You don't say if it is just the boat trip you want, or the trip up to Voss and railway to Flam as well.

If just the boat trip there is a bus back to Flam direct from Gudvangen via the road tunnel at 1347.

If the rail trip as well then your train both from Voss to Myrdal then down to Flam are very off peak to the direction of peak travel- both the Norway in a Nutshellers and any cruise traffic.

It is often forgotten on this forum that the trains are not tourist trains, but local trains used by local people, who do not book months and months ahead of time.

Thanks, we're going the Sognefjord route to Oslo so have that covered. Picking up the Naerfjord this way all we'll be missing is the Bergen - Gudvagen rail/bus route.