The Italian ones are nice, as are the UK ones (if a bit cramped). The Nightjets mentioned in the article (mostly taken over from DB's CityNightLine a few years ago) are variable; the Poland-Ukraine one is pleasant enough but poorly timed. I had a pretty bad time on a Hungarian Railways one. The Russian Railways ones from Paris and Nice to Moscow have a reputation for luxurious looks but bad (or at least not westerner-friendly) food.

Austrian ÖBB, which runs in multiple countries like most lines going through Germany, is working on modernizing their trains, but they also have quite old cars. Not sure if there is a good site showing car types and train lines. https://www.nightjet.com/en/komfortkategorien/nightjetzukunf...

I was on that site a couple of times but never managed to find out which routes offer a single cabin for yourself.

I would not want to have strangers sleeping next to me.

During Cornona this probably is different, but here's the relevant page: https://www.nightjet.com/en/komfortkategorien/ganzes-abteil-...

Well, that is a page which describes the options.

Bit have you tried the "Book ticket" button?

For me, it simply goes to the start page.

So I still don't know how to find the routes that offer these options.

https://www.seat61.com/trains-and-routes/nightjet.htm suggests that (nearly?) every route offers a single compartment.

That is, "Most Nightjet trains use Comfortline sleeping-cars built by Siemens in 2003-2005 for German Railways' City Night Line sleeper trains." and "Below: Comfortline sleeper layout. All compartments can be sold as a single, double or triple."

Well, have you tried it?

Every time I try to book a trip, the single compartment is grey and says that this option is not available for this trip.

When did you try?

I picked 14 April 2021, leaving Wien Hvf (U) at 20:13 arriving Berlin at 08:55. "Sparschiene Nachtverkehr Nightjet + Anschlussticket Nightjet." "Sleeper Bed", "Compartment with 1 bed (Single)".

€ 216,00.

Bear in mind the comment at https://www.nightjet.com/en/ : "Due to the travel restrictions caused by the corona pandemic, we are reducing the Nightjet traffic to probably 8 February 2021 ... As a precautionary measure, no bookings are currently possible for the affected trains from January 10th to March 24th, 2021", confirming johannes1234321's comment "During Cornona this probably is different".

Almost all of them. Certainly at least Cologne-Vienna, Basel-Vienna, and Amsterdam-Munich back when it existed. What ones did you try?