Which accommodation on which night train ? Generally couchettes and sleepers can be used with a 1st or 2nd class pass, price is the same. Seating carriages can be 1st or 2nd class or have no classes at all.

In any case you can travel in 2nd class with your 1st class pass, that's not an issue.

Thanks. For example: I want to take the euronight train from Lugano to Ljubljana (EN 40465). Is there a first class compartment? Or do I have to take a seat, sleeper or couchette second class reservation? (I have a first class pass)

So this night train runs from Zurich to Zagreb and indeed stops in Ljubljana. It has reclining seats, couchettes and sleepers (class undefined).

To book this night train go to nightjet.com or tickets.oebb.at and add Eurail as a discount. It'll better show prices and availability than eurail.com. You also don't pay booking fees.

If you have another question, mention travel date and we'll gladly help.

Thanks for info!