You can board the train with a valid ticket; a seat reservation seems not to be required. However, you might not get a seat in this train, that’s why usually you are still able to make a reservation even when not required. I don’t know though, if the seats are already sold out or if the prices can’t be shown due to technical issues from Interrail.

Hope I can help :)

Sometimes it helps to split your journey in two parts? I recently also had troubles reserving for an EC from Zagreb, I split the reservation in two parts, with an interchange in Villach. Downside is a bit of an extra cost.

Although a lot of night trains seem very popular this summer, I read an article that almost everything was fully booked.

NJ=NightJet have in principle only RESeved seats/beds. The sleepers/beds are nearly all sold out for summer (yesterday in the news here)-there are seats-a few or a lot-depending on route. It used to be the case that IN Asutria only a SEAT-RES was not needed, but the real risk is as above and also that they will not let you come aboard or send you off next stop.

It is -if still available-fairly easy to REServe on oebb.at-do not choose ´seat only´ but full ticket-then choose reduction (small extra pop up)-its mostly 14/15€ extra for seat. It says you have to print/collect ticket in AT-but thats not really the case for just REServ as log as you can also show it on phone

This train has mandatory reservations and is fully booked.

So it is not possible to buy seat reservations at the train station, in Zurich? 

Nope fully booked means what it is. No seats/berths free anymore Maybe you have luck and somebody cancel it´s booking but i doubt that.

No, it is full for the departure on 10 July. There are still seats (and also couchettes) for 1 day later (11 July) or 3 days earlier (7 July). You can book them via ÖBB: do NOT use "Seat only” but add the "Interrail / Eurail” discount and then look for normal tickets.

Oh okayy so when on the website says that it is not avalibale, that means that its fully booked? 

That is one of the causes of this message. Another frequent one is that there is a connection problem to the railway company's reservation system. I just checked on ÖBB.

I would be great to know which problem causes the messagebut thank you for your answer

I’m looking to book this for 23rd from Zagreb arriving Zurich 24th. I can’t see the option to get Interrail discounts though (and as implied above the “seat only” option doesn’t work or is full). All I can see is the full fare for €89.

Seats are available for 14 € per person. Couchette and sleeper are sold out.