check via bahn.com if reservation is possible, plus they show if the train will be full or not.

Reservation cost via bahn.com 4,50€ in 2nd class/journey (5,90€ in 1st). Via oebb.at 3€/train. Via NS International should it also be possible. 

If all seats are full you have to stand. If it is to full you have to leave the train. 

Interrail.eu is not reliable for this things. Always check on the website of the train company that runs the train. 

Most likely this message is from bahn.de? 

You can board-as you likely know train is not plane and noone will do check-inns etc-but there is a fair chancde that somewhre along the route all standees or even those without RESev are asked to leave-and maybe get 20/30€ compensation. 

Het is dus echt niet anders als bij NS-niemand checkt hoeveel er in stappen, maar als de ´veiligheid´ in gevaar dreigt te komen, dan komen er maatregelen.

Overigens is dit 1 trein/dag die doorloopt-wees alvast wat wijzer en bekijk dat op het overgrote deel traject elke 20/30 min andere treinen rijden. Dat zijn zo van die vanzelfsprekende dingen die niemand meer lijkt te kunnen bergijpen sinds al die planners alles voorstippelen

It says reservations are not mandatory. However, can I still make a reservation to guarantee my seat? I’m not sure how I’d do this and use my eurail so it’s at a discounted price. 

Yes you can. For all reservations in Germany, Austria and many other countries :

- 3€ per train on tickets.oebb.at (free with a 1st class pass). Add Eurail as a discount and select one-way tickets

- 4.50€ per journey on bahn.com

Eurail optional seat reservations are way more expensive at 8€ so don't make them there.

Thank you! For the sleeper, I’d like to make a reservation (and my ticket will be with my 2nd class Eurail Global ticket travel day). 
For the reservation I make, will it be possible to make it for a bed rather than a seat? On DB, I only see Open Saloon or Compartment options and I’m not sure if either of these is a sleeper. 

For couchettes or sleepers use tickets.oebb.at or nightjet.com

This night train is very popular and most of the summer is already sold out. What would be your travel date ?

I’m planning to take the train from Amsterdam to Zurich from 20:28 to 8:05 on June 2, 2023. 
Ok if I understand correctly, it’s possible to make a seat reservation for a seat, but may be difficult to reserve a sleeper because they’re booked out earlier?

You're really lucky ! There are a few couchettes left to Basel (arrival 06:20) and you then move to the seating carriages until Zurich (no need to use a second pass day).

- 34€ couchette in a 6-people compartment

- 44€ couchette in a 4-people compartment

Book ASAP on nightjet.com (or tickets.oebb.at). Add Eurail as a discount

night trains reservations can't be booked via Deutsche Bahn, only via Austrian Railways ÖBB using the one way ticket option and Interrail/Eurail as discount.

Keep in mind that the Nightjet and Euronight services from/to Switzerland to Amsterdam/Hamburg/Berlin/Prague have also seats they do not run as Nightjet/Euronight but as Intercity/Eurocity with optional reservations (but highly reccomended).

Sounds great! Thank you so much for the help, this is my first trip and there’s definitely been a learning curve. 
Just a few clarifications: If I book a sleeper to Basel, do I still need to book a seat reservation (3 euro) to Zurich? And would it only be one travel day since I’m on the same train the whole time?

If you book a sleeper to Basel you don’t need to make a seat reservation from Basel to Zürich.

@Cadbury007 yes exactly it would only count as one travel day as you're staying on the same train. Add the train between Amsterdam and Zurich to the app (before boarding) so it only uses a travel day

To add : seat reservations are completely unnecessary in Switzerland, nobody makes them. :)

Let us know if something doesn't work

No reservation needed and it is important that you  enter the whole travel from Amsterdam to Zürich as one journey in the Railplanner app. If you enter Basel-Zürich as a separate journey, that would use a new travel day.