Sleeper trains are doing fine in Central and Eastern Europe, despite having to contend with some long running times. Sweden is trying to revive them through Denmark, which hasn't hosted night trains since 2014 [1].
There's two viable, existing routes for trains between the Central Sweden and Northern Germany. One's through the Fehmarn Belt rail ferry, and another is the long way through Jutland. Both ways take a long time compared to the distance covered. Stockholm is already ~5 hours by fast train from Copenhagen -- this segment of the journey stays the same. It's after that things get complicated and slow. Operational issues remain with changing voltages and train protection systems (ETCS Level 2 will be built out in a few years), or with the ferry.
A Fehmarn Belt fixed link would be a boon for such a service, like the article says. But it remains to be seen whether that will be built.
Re: Central and Eastern Europe, I took an overnight (sort of, 5AM to 5PM) sleeper train from Lviv to Budapest a couple years ago. It was quite pleasant and they managed the journey in a way I never would have expected. Instead of passenger layovers and shuffling between trains at subsequent stations, they just designated a whole car bound for Budapest and then detached & reattached it to different trains along the way. Made the long ride a surprisingly pleasant and restful experience.
They're called though coaches (Kurswagen in German) and used to be quite common. In fact, quite a few night trains are split and recombined during the night, but usually in chunks of three or four cars.
One of my most pleasant travel snooze memories is waking up in a very comfortable bunk bed, looking out the window to the morning sun rising over the Danube, a castle and some vineyards and fruit gardens rolling off into the distance, slowly coming to a two hour halt and having a mountain valley full of green life splayed out for the breakfast picture.
If I remember correctly, a night train from Stockholm to south Europe implies a change of train in the middle of the night in either Malmö or Copenhagen - which sort of ruins the the concept of sleeper trains.
I don't see why that would be necessary. They might have to switch locomotives a couple of times during the journey, but they can just do that with the same set of carriages, with the passengers sound asleep during the switches.
There's two viable, existing routes for trains between the Central Sweden and Northern Germany. One's through the Fehmarn Belt rail ferry, and another is the long way through Jutland. Both ways take a long time compared to the distance covered. Stockholm is already ~5 hours by fast train from Copenhagen -- this segment of the journey stays the same. It's after that things get complicated and slow. Operational issues remain with changing voltages and train protection systems (ETCS Level 2 will be built out in a few years), or with the ferry.
A Fehmarn Belt fixed link would be a boon for such a service, like the article says. But it remains to be seen whether that will be built.
[1] https://back-on-track.eu/night-trains-to-europe-new-opportun...