As a veteran of the great Trans-Mongolian/Trans-Siberian train adventure, I have pretty high standards for scenic train rides 🙂 Definitely, the train ride from Split to Zagreb is one of the best!
The train itself, run by Croatian Raiways (HZPP), is tiny, with just two coaches. Overall, it is a 6.5hr train ride, through coastal Split, mountains, plains, and the suburbs (of Zagreb). I bought myself an (overpriced :-)) Beer and enjoyed the train ride as much as I could. I also met some American and British students doing a weekend trip to Zagreb. The train moves quite fast considering that it has to traverse some high terrain.