RailNinja is not a website to be trusted. Interrail doesn’t show availability until you’re logged in (it only lists prices).
SJ is the best and most reliable website. I’ll have a look and update below, it’s totally likely that the night train is sold out.
Vy (the operator, on vy.se) also say that train is fully booked, so that is likely to be the case.
Interrail can't book it either for that date. If you try to get to the payment page, then you'll see a message "Sorry, this connection can't be booked right now. Please try again later or find another connection. This connection will be removed from your cart.”
I suspect that booking via Railninja will fail when you try.
Additionally, because of the recent derailment, it may not run all the way to Narvik. According to Vy, there is bus replacement between Kiruna and Narvik until at least 1/1/2024. But the train is also fully booked between Stockholm and Kiruna.
So the night train to Narvik is completely full on most January dates. It’s a really popular route and trains are infrequent so it doesn’t surprise me much. Here are a few alternatives :
OPTION 1 via Boden (2 pass days) :
- night train Stockholm C - Boden C 21:55 - 10:51 2020 SEK (157£) for 2 private compartments (berth numbers at check out) → even better than a cramped compartment for 4
- R Boden C - Kiruna 11:45 - 15:00 220 SEK for 4 seats
- Kiruna - Narvik : see rvdborgt’s post
There are cheaper couchette compartments available but you’ll likely have to share with 2 other people. Not possible to book a private couchette compartment with Interrail AFAIK.
EDIT : thanks a lot rvdborgt for the heads up about the derailment. Vy says that the railway is closed until 1st January at the earliest and that the repair is going to take time. This leads me to think that I’d buy the night train + Boden - Kiruna train (as soon as possible). Then you should be able to board the 15:32 bus.
Maybe call Vy tomorrow ?
Let me check if there’s something else in case. Would it be possible to travel via Norway ?
OPTION 2 (suboptimal) :
- X2000 Stockholm C - Sundsvall C 09:21 - 12:56 260 SEK total
- R Sundsvall C - Storlien 14:10 - 18:25 272 SEK total
- R Storlien - Trondheim S 18:49 - 20:28
- night train Trondheim S - Bodø 23:05 - 09:05 (you can board earlier)
- bus to Narvik in the morning
I cannot get prices for the night train but expect something similar to the Swedish one. entur.no doesn’t work for multiple passholders at the same time...
I repeat myself but I’m pretty sure that option 1 is the way to go. Call Vy if they can help regarding the replacement bus.