Yes, train is reliable option but will not take you further north than Bodų (which is quite a distance north anyway).
For northern light I was thinking about going to the region called Finnmark and maybe stay over in one of those ice hotels (I think it's in Alta) and combine with some dog-sled "safari".
Another option was to go to Tromsų and see northern lights from some arranged tour there.
I found a lot of info when googling northern lights in Norway some time ago.