All types can be booked through the OBB Nightjet website. If you are using interrail passes then you can add that as a discount for each traveller and the prices will adjust accordingly.

Berths in couchettes and sleepers as well as sole use of sleepers are given discounts, sole use of couchettes compartments do not.

https://www.nightjet.com/en

If you book all together in one transaction then you will get put in the same compartment if possible, if you book single berths seperately then you are all but guaranteed to be in different compartments.

Depending on how many of you there are you can get full occupancy of a 4 or 6 person couchette compartment just by booking 4 or 6 persons.

@Eamon T 
Have a look on the 

post. Here you can find manuals for the Nighttrains of OBB like Berlin - Vienna - (Graz) 

And for the daytrains from Naples to Milan 

For Italian domestic Nighttrainreservations with Interrail you need to visit a Italian Ticketoffice :/  And in my experience a Super Eco or Eco fare work usually cheaper than a Interrailtravelday + Reservation 

Eg. I paid just 79€ for a private Sleeper from Palermo all the way up to Milan :) 

Belgian railways also sell reservations for Italian night train. You can book them by phone and they'll send it by email. Alternatively, you can book via DB by phone and collect reservations in Germany or have them sent via post for €5.90 per order.

AFAIK, only for a private sleeper compartment the Economy fares can be cheaper than a pass reservation, the latter one being quite expensive for a single sleeper.

Trying to find out how to book/reserve coupe/compartment on a night train from Frankfurt to Rimini with Deutche Bahn. Does anyone know how to do it? I only reach the standard reservations with regular prices and cant seem to find how to “connect” the reservation to our Interrail.

You can't book night train reservations online with DB.

There's no night train from Frankfurt to Rimini but there's one from Munich to Rimini this summer (10/6-9/9). It can be booked on tickets.oebb.at. Do NOT use "Seat reservations only” but add the Interrail/Eurail discount and then select "One-way tickets and day tickets”. If it doesn't work for you then please mention your travel date.

Of course its Munich to Rimini, I mixed two cities of departure!

I found my way to “One-way tickets and day tickets” but cant find where to add Interrail discount. If I look at the step which is for passengers there is a button for “add discount” but they are all in German and I cant guess which is for Interrail. Is it the right place to add discounts as this?

Travel day is 21th of June at 20:09.

There's a search box for the discounts. Start typing "Interrail” and it should appear.

Thank you for the help, found it and got the tickets!

Just one last question: When I got the ticket in PDF it only said 2 beds. Our child is 2,5 years and thatt must mean she shares bed with one of us?

That's correct. If you want your child to have a separate bed, then you should enter the age as 6.

You can cancel the booking (100% refundable until 15 days before the journey) and book again for 3.

As I said, thank you for all the help! It made it much easier to make the reservations.

My best, 

Peter