Swedish Railways runs some lovely sleepers which are, I think, the best way to get up to the north; and when you factor in the cost of a hotel room, they are also as cheap as flying and spending the night. What I learned this trip is that it is more comfortable, as well as cheaper, to take an entire three-bed second class carriage to yourself than to book a single-berth first class carriage. This is because the first-class carriage squeezes in the world’s smallest shower and loo; both are too small, especially the shower, and this means that everything else in the compartment is also too small. Not worth the extra money. In second class, you have to walk down the corridor for these amenities, but the carriage is much more spacious, and the tracks up north, unlike the ones to Östersund the last time I took that train, are really quiet and conducive to deep sleep.
One nice touch: as we were pulling out of Luleå I realised that I had left my English mobile charging in the ticket hall and the conductor stopped the whole train so I could run back and get it. Compare this to the archetypal Stockholm experience of running onto the tube platform to see the driver shutting the doors just as you arrive at them, and then watching your gesticulations curiously before turning to face ahead again and pulling out of the station.