Here is some information about the new night train that SJ will start running from Stockholm to Hamburg Altona on the 1st of September.
The information webpage is unfortunately only available in Swedish, but there’s always Google translate to assist.
https://www.sj.se/sv/reseinfo/sj-euronight.html
The train has seats, couchettes and sleepers. The price for a seat is from 155 SEK, a place in a 6-bed couchette is from 363 SEK and a place in a 4-bed couchette is from 467 SEK. A place in a 2-bed second class sleeper with a breakfast box is from 779 SEK. The price seems to be the same for a non-refundable and a refundable reservation.
https://www.sj.se/en/home.html#/ Here you can make the reservation. Select “Interrial or Eurail” as railpass. As Pass Cover Number an I or E followed by any 8 digit combination will do.
Right now there is a bug in the system that only gives the Interrail price on the first reservation if you make a return reservation in the same booking. I spotted that today and have reported that to SJ who will look into it.