Shouldn't be a problem to board in Zurich. Simply tell the staff before boarding, just in case.

The reservation prices are the same anyway.

I’ve asked the question of ÖBB via twitter. I don’t want to pay 32 euro fee for that journey if possible 

Well you already have the reservation for Buchs SG - Zagreb anyway, no ?

I mean : as they cost the same price, staff won't bother if you board early. But still tell them on the platform

If you have an Interrail pass there is no need to pay anything for a journey from Zürich to Buchs SG in a seating car. 

OBB says we have to get an extra ticket - the planner is showing the 20:40 train as requiring a 16 euro reservation fee to get on at Zurich towards Buchs SG

Normaly Nightjet staff has no problem if you enter before the train. But I can say it 100% sure about everyone of the staff.

What time does the sleeper train get into Zurich?  Is it the starting point for the journey. I’m thinking we could ask to board and if not take a different train to get to Buchs SG

Everything is available on planners such as SBB app, sbb.ch, DB app or bahn.com.

Yes it is the departure station so the train will arrive about 20 min before departure

- EN Zürich HB - Buchs SG 20:40 - 22:05

Alternative :

- IC3 Zürich HB - Sargans 20:38 - 21:33

- RE Sargans - Buchs SG 21:39 - 21:50

But I'm 99% certain that you'll be allowed to board.

Thank you this gives us a good option if NJ staff to let us on the train at Zurich. Is the journey between Zurich and Buchs SG scenic?

can I just check is the train to Sargans the one at 20:35?

Yes the journey is quite scenic, esp. along Walensee. Fortunately in summer there's some daylight left.

Which 20:35 train ? The 20:38 train goes to Chur and stops at Sargans and Landquart first. The connection in Sargans is easy and guaranteed.

It’s IC585 showing in the planner app  that shows it departing Zurich at 20:35

Please never use the rail planner app to plan. It doesn't get enough updates to be reliable.

Ok thanks for the tip.  Should I use ÖBB instead? 

There are engineering works this weekend and IC3 (line number) or IC585 (train number, which people rarely use, at least in Switzerland) indeed departs at 20:35.

It’s best to use the railway company website, in this case sbb.ch or SBB app. The app will be very useful for connections, train compositions, platforms and eventual delays.

Bahn.com is also very reliable, all across Europe.

Will any of our intended routes be affected at this stage?  Thanks for all your help

The online timetable has been updated taking the works into account, whether on Rail Planner or on SBB app/sbb.ch.

However for reliable on-time information, I recommend using SBB app (or the website) :)

EDIT : are you coming from Munich that day or not ? If so the EC leaving Munich at 16:52 and arriving in Zurich at 20:27 is often heavily delayed (25-30 min). All EC trains on this route are unfortunately very unreliable : there is a single track for most of the German route so trains need to wait for any incoming train. I would suggest another connection (the EC leaving 2h earlier for example) to play safe.

We will probably leave Munich in the morning and spend half a day in Zurich which hopefully will take some of the stress out of the journey

This Abel thank you for all your advice. They let us on at Zurich so a lot of stress and €32 euro better off.   You could have warned me how terrible Croatian trains were tho!