Depends on how much of cold you can tolerate-and how cold it will be-climate warming is also there. PLUS: do not forget the long+dark nights: around 21/12 it is dark all day above polar circle and in cities like STO or HEL only light from around 11-15. But as such perfectly possible-over there they are very well used to winter, ice+snow etc-and also they do not strike that often as in FR.

IN winter from Lit to PL also by bus-the only weekends direct train only runs summer and has stopped already. You can pick up train again from Suwalki in PL_but that will lengthen trip time a big lot-stay on bus till Warszawa.

Having just been to DK+SE 2 weeks ago I would suggest to not just stay in the capitals. And save-its even more expensive as along the mediterranee in hi-summer.

There as a recent same question about doing Kiru-Rova in 1 day-it seems possible with a VERY early start-only mo-fr, and part by bus (probably also Boden-Haparanda=border village, even though there are now again a few trains) and always from other FI side=Tornea to Kemi. Walk over border to busstop in FI (about 1,5 KM).

And try to avoid staying there on Xmas=25/12-all+each closed and super expensive special holiday prices.

@mcadv No it hasn't stopped, as we have told you numerous times!

It will run weekends only until 10 December. From 11 December there will be a daily service, timetable to be confirmed:

https://www.seat61.com/trains-and-routes/warsaw-to-vilnius-by-train.htm

I am planning a similar train tour from Hamburg /where I livе, there is a direct night train from Hamburg-Altona to Stockholm in only 12 h travel time/ then from Stockholm to the north with stops in Luleå, Galivare or Kiruna, Abisko and Narvik. In mid-February, the day should already be as long there as it is in Hamburg at the beginning of January, and the winter is still harsh and snowy at that time. In my opinion, the best time to experience the true north is in winter, before the snow melts in March.

How many interrail days will it be if I travel on the direct train at 21:50 from Hamburg on day 1, arrive in Stockholm on day 2, and from there take another direct train to Luleå on day 3 early in the morning? Does this count as 3 days or 2 days interrail?

All days (00.00-23.59 CET) when you board a train count as a travel day so that will be 2 travel days. 

There are no direct day trains from Stockholm to Luleå. There are direct night trains.

It is possible to travel during day time but then you need to change trains once or more.

I mean the direct night train to Luleå.