Thankfully, I managed to experience the westbound Jan Kiepura a few weeks back. Was a lovely experience, but can see why the train is perhaps struggling to pay - only 3 through cars (1 x sleeper, 1 x couchette, 1 x seated), perhaps 40-50% loaded at best.
That's quite sad - I remember as recently as the late 1990s these trains on the trunk sections being huge, long loco-hauled rakes of 12 or more coaches. A once proud tradition.
I guess the difference from the UK ones, other than the subsidy, is that air doesn't compete well with most of the UK ones - there is near no competition (other than day trains or driving) with the Penzance one and the Fort William one is the only way to do a weekend in the mountains of the area from London full stop (the timings don't work by any other mode of transport unless you are happy to risk driving through the night). There are plenty of established flights from London to Glasgow/Edinburgh/Aberdeen/Inverness, but none of them (that I know of) allow a 9am arrival in the office even with an early start.
They are also greatly attractive to tourists, which the European ones are far less so, except InterRailers in the seats (which might explain why DB are not pulling out completely from overnight trains, but instead will be operating seated-only ICs and ICEs overnight on selected routes). And Germany is presently having a love affair with the coach, with regard to which, while coach travel is well-established and successful in the UK, most people would consider it a fate worse than death for a London to Scotland slog.
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PKP got out of operating the sleeper independently a couple of years ago by hanging it onto a regular EC East of Berlin and the two CD services from Prague only made sense in combination with other portions. IIRC CD will run an evening Prague-Leipzig and morning back to fill in the gap for daytime travellers - presumably using one of the EC sets in marginal time. There's also speculation about a Prague-Zürich sleeper via Linz, attaching to EN466/7 which ÖBB is happy to keep running.
To be fair the ECs east of Berlin are operated by PKP locomotives and stock (possibly DB staff though[1]), though costs may be shared with DB. The joint "Berlin Warszawa Express" branding seems to be dying off.
I do find OeBB's continuing interest odd. Is it a cultural thing - do Austrians feel more inclined to use night trains than Germans? It's a very small country that otherwise wouldn't seem to have any need for them.
[1] I've used the service once during the DB strike, so it only operated within Poland with a poorly-advertised[2] replacement bus on the German section.
[2] Not in the planner, not at Berlin Hbf, not mentioned anywhere else. You had to know about it. Which is why I didn't, and instead started at 4am for a string of local trains, only to find when I reached the train at the Polish border that it then waited an hour for the bus to arrive, again with no information whatsoever until about 10 minutes before it went. Was an interesting trip and I enjoyed it, but it was also a fine example of how
not to do passenger information.