Late to the thread, thanks for the mention
@30907. It sounds like a great trip.
I live near Umeå around 64°N. So depending which train you take you will probably be asleep when you roll past my place
However, a few important comments.
Sweden's rail system has had a terrible summer, with surging demand for train travel, heightened demand because of this year's Interrail deals, and the usual reduction of services in the summertime due to the Swedish culture of taking long summer holidays in one block. SJ, Vy and many other rail operators have struggled to roster train drivers and crew even for the reduced summer timetable.
However today is the first day back at work for many Swedes, and most rail and bus systems are ramping back up to normal schedules.
However, many many many long distance trains into and in Sweden have been fully booked for weeks. If at any point in your trip you find yourself unable to get a reservation (and I really do recommend a private compartment on the night trains, it's worth the comfort and the better sleep quality) remember that as suggested up-thread you can piece together an itinerary using local trains. It is quite possible to travel across Sweden only using local trains, and these are always unreserved. You will have to make many more changes and you won't have such good on board dining options (but often coffee from the conductor's kiosk), but you will have a lot more freedom. This is also the only way to travel with a bicycle by train in Sweden: SJ and other long distance operators don't accept bicycles.
Regarding Haparanda / Tornio... there are three trains a day from Luleå and/or Boden to Haparanda. If you are coming from the south you can change in Boden. Haparanda station is technically capable of a cross-platform interchange between Swedish and Finnish trains, but that will still take a few more years. When you arrive in Haparanda, you can walk to Tornio-itäinen station in about 30-60 minutes, depending on your mobility. There is only limited service: namely the less-than-daily night train between Kolari and Helsinki, which goes south in the evening and north in the morning. Alternatively, you need to walk across the border and head towards the centre of Tornio to catch a bus towards Kemi or Oulu. (Most of these buses start here, not the fancier Haparanda-Tornio bus station on the Swedish side of the border). There are many many different small private bus companies in Finland, but most can be booked through the
Matkahuolto website.
Good luck and enjoy your trip.