You should be able to book it on sj.se, adding the interrail discount

What date did you look for? I can make the reservation. 

You can’t make reservations from the app. And don’t book at Interrails own booking site if you can avoid it - it adds a €2 booking fee.

Reservations for SJ trains is best booked at sj.se. They have just changed their booking system for journeys after June 4. So things looks a little different from som guides you might find (Like this one: https://www.interrail.eu/en/book-reservations/how-do-i-book-seats.

Here is my instructions for the new SJ booking system. This might also be useful for other users:

• Go to https://www.sj.se/. Or  https://www.sj.se/en for the English version (These instructions will be based on the English version).
• Click “Search journey”.
• Choose travle date. This is to determine if you will be using the old or new system. This step might disappear after June 4.
• Select your departure and arrival stations. You can also change departure date again.
• Now you should tell the system that you have an Interrail/Eurail pass and therefore only need a reservation. This step has changed a little from the old system: Click “Customise” next to “Traveller 1”.
• Click “Travels with travel card”. A dropdown menu appears. Select “Interrail or Eurail”.
• Enter your pass number in “Card number”. It currently looks like there is no validation. For test purposes you can enter anything, but for a real reservation you should enter a valid pass number, since it is printed on the final ticket.
•  Click “Save”
• Add more travelers if needed.
• Choose your journey.
• Choose travel class. 1. og 2. class for day trains. Seat, couchette or sleeping compartment for night trains. Make sure the pass is valid for the class.
• Choose private or shared compartment.
• For shared compartment, choose between men’s, women’s or mixed compartments.
• Choose flexibility. “Non-rebookable” tickets will be full price, so just choose “Refundable”.
• Click “Continue”.
• Unlike the old system, there is no option to choose between upper, middle or lower bed/berth. I hope this will be fixed. If needed, keep trying in new browser tabs until you get the type of bed you want. If you book a shared compartment from Copenhagen at 00:10, I suggest you try to get a lower bed, to disturb the other passengers as little as possible - they might already be sleeping.
• Click “Go to checkout”.
• Enter name(s), Contact details and payment method and click “Pay”

Palle Dam

There is now a validation and you need a valid pass number, preferably a 6 character mobile pass number. 

AFAIK you can't book a private compartment with SJ if you have Interrail. 

Don’t know if is just for me or a general new issue:

When entering the interrail passnumber, it is not correctly validated. One cannot continue the booking.

I also tested with a valid passnumber and a valid mobile pass number.

As of now, no booking on sj.se is possible with interrail.

Works for me, with a mobile pass number (6 characters).

Huh ok. I have another pass, which is also working. Both are bought the same day, but one is already activated for the travel time in november, the other one is not. I guess I’ll write to sj and see what the say.

Thanks for testing!

Last time I booked with SJ, the pass number didn't appear on the reservation. They've changed the backend booking system though, so I don't know if that's still the case. Maybe @AnnaB knows?

Did sj policy changed recently? No Eurail or Interail discount on private compartment. Would have loved to know that before buying Eurail pass. Waste of money.

I can book a private compartment with shower/wc (Hamburg-Stockholm on 12/11):

For the private compartment without shower/wc in the compartment (which they call 2nd class), no Interrail/Eurail fare is indicated, which is very strange if you CAN book the more luxurious compartment with Interrail/Eurail.

I'd ask them via social media, e.g. Facebook, what the problem is and how you can book.

I don't think that you can book a private 2nd class sleeper with Interrail/Eurail. 

Why not when you can book a “1st class” private sleeper?