Entering INTERRAIL or EURAIL as a code on snalltaget.se when trying to book a night travel from Hamburg to Malmö does not have any effect: Is the Interrail pass actually not valid on that one?
Looks like you can only book trip and seat/bed together.
I read in this forum somewhere that booking on the Snälltaget within Sweden is possible like that, like from Malmö to Are!