I did this in Russia and Ukraine. The bathrooms were disgusting and it was actually scary due to the seedy characters onboard and the lack of security.
www.willerexpress.com/ Booking in English,Korean and Chinese.You can pay by credit card.
www.eurorailways.com/Official_Site Wake up at your destination, night trains tickets. Book Now!
www.zagreb-touristinfo.hr/ Begin with Zagreb, Croatia - Old Metropolis in the Heart of Europe