Thanks to RDC for taking over from SJ
RDC took over this route (Berlin-Hamburg-Stockholm) after SJ unceremoniously withdrew their subsidy so kudos to them for that, and for negotiating with Interrail to keep an affordable pass-holder couchette offer, €37 (though the sleeper costs 85!)
Same characterful trains and staff, and mine arrived spot on time in Stockholm in mid September.
An improvement would be if, when booking, one could specifty top or bottom bunk. A person over 6ft can't fit in the top; and a person with an impairment probably would need the bottom. Even better if you could specify a particular carriage and berth BEFORE paying so that friends aren't split up when travelling together. The SJ app and website allowed this. Finally, a more flexible cancellation policy is required as connecting trains up to Hamburg are often late and DB won't cover your loss of the RDC reservation as it's a different company!