Hey! In August I will be traveling with Interrail for the first time.  But now I am a bit confused when I try to book a seat. The train I try to book is a night train running from Zagreb Glavni Kol. to Split. However, I prefer a couchette but I see that this is not an option. So I am guessing this is just a seat right? If so, does anyone know where I can book a night train that actually has a couchette? 

Thank you!