This can be booked at https://www.nightjet.com/ train number is EN 50466

Add Interrail/Eurail as discount for each passenger and search to get passholder fare

Cost for 2 in private sleeper is €116 or €156 for en-suite sleeper.

However it is too early to book for May, bookings for that train are currently only available to Jan.

Seat61 has a great summary of all the reservation costs, and how/ where to reserve, on this page: 

https://www.seat61.com/interrail-and-eurail-reservations.htm

Prague to Zurich by EuroNight sleeper train (both routes):

With a 2nd class pass, bed in 3-berth sleeper €45, bed in 2-berth sleeper €58, bed in single-berth sleeper €110.  With a 1st class pass, bed in 3-berth deluxe sleeper with shower & toilet €45, bed in 2-berth deluxe sleeper with shower & toilet €58, bed in single-bed deluxe sleeper with shower & toilet €110.

Thanks for the web site, train number and info on how to apply the Eurail pass. “bed in 2-berth deluxe sleeper with shower & toilet €58” sounds good. “it is too early to book for May” is puzzling. I would have thought they would take bookings well in advance. Does this mean it becomes a mad scramble for “first in first served” when they finally make the openings available? Any idea when that’s likely to be?  

You can search for your journey on www,bahn.com - the best website generally for train timetables in Europe - and click through to the ‘Details’ section where it will show all the stops a train will be making. That will confirm that EN50466 is going via Austria.

Thanks for elaborating on how to figure out to plan the right train. If I can get this one figured out, I should be able to do the rest without much help, I hope. Any idea when the May schedule is likely to be released?

Yes, they generally open as much as 6 months ahead. But often the 10 December timetable change gets in the way.

I suggest you keep an eye on it!

These trains do get booked up. but I doubt you’ll have a problem if you keep checking. Unfortunately I couldn’t give a guess as to when May will be released.

Thanks

Does this mean I am too late and there are no night trains? Is there anything I can do? If there’s no night train, I need to re-configure my entire holiday.

No, don’t worry. There’s probable some uncertainty in the schedule (probably some engineering works), so they didn’t open booking yet.

You can ask via the socials or customer service of nightjet when the bookings will be +/- opened.

Thanks, I’ve asked the question of NightJet. We’ll see if they answer. 

Would I be safer getting Eurail to make the booking for me. I know they charge a fee, though I don’t know how much that would be. However the outcome of this one is fairly important. If I can’t get that night train, I will probably drop Prague from my itinerary and re-schedule the rest of my trip.

Eurail charge a €2 booking fee per person and train. But you won't be able to book it sooner through Eurail than through Nightjet/ÖBB.

Thanks.