[TooCreative]

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.

[eesmith]

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."

[TooCreative]

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.