It will cost 1 travel day, on 6 January, including an inbound/outbound.

All trains - sleeper or day train - count on the day at the scheduled time (CET) of departure.

So this will use 6 Jan, and you can take other trains on the 6 Jan if necessary.

Previous years Interrail has not been valid on the Snälltåget skitrain to/from München and the Austrian alps so there must have been a change if you have managed to get an Interrail reservation on that train.

I tried to verify availability on Snälltåget's website but you can't currently book pass reservations there; they say you have to email them.

I'd be curious to know whether Interrail can be used on that train.

I couldn't find the info in English, but there is only information about Interrail reservations within Sweden, and between Sweden and Berlin on the Snälltåget website. The price for going to Hamburg is the same as for going to Berlin.

Re Snälltåget's website : Yeah this has changed with their new booking interface last month. Too bad that it goes in the wrong direction… instead of favouring passholder purchases.

It is only temporary that you need to contact Snälltåget customer service for the Interrail reservations. Apparently it has something to do with an update of their booking system. Hopefully sooner, and not later, it will be possible to make Interrail reservations through the Snälltåget website again. 

Tried to make a reservation several times today. Not possible. Snälltaget linked me to a Phone number that doesnt work at all. Will take instead the Deutsche Bahn train one hour earlier now.