prague to croatia to greece

To learn more about trains in this region, I'd start with The Man In Seat 61. Here's his page on getting to Greece: http://www.seat61.com/Greece.htm#.U3fEpCiGd8E. The map on that page will show you some other routes through the region. And on the right of that page, you'll see his links to other countries (like Czech Republic and Croatia).

Split is a "dead end" for trains, so you'd have to go back to Zagreb to connect from Split to other destinations.

To look at schedules, use the Bahn website, following Rick's tutorial: http://www.ricksteves.com/plan/tips/db_tips.htm

From Prague to Croatia would be an extremely long train ride--at least 20 hours to Split, and possibly up to 24 hours. I actually recommend looking into flying in order to make the best use of your precious travel time. We recommend using www.skyscanner.com for finding flights in Europe. Skyscanner will show you flights offered by commercial airlines as well as budget airlines.

Visiting Croatia, I would not want to miss Dubrovnik. At this link you can see the locations Rick recommends visiting depending on time constraints (the recommendation includes Split depending on how much time you have): https://www.ricksteves.com/europe/slovenia-croatia-itinerary

For getting to Greece, I would also recommend flying in order to save time. Again, Skyscanner is the recommended site for checking on flight options.

And here is a link that will help you to plan and prioritize your time in Greece: https://www.ricksteves.com/europe/greece/itinerary

The only practical way to do that trip is to fly. That region is not well connected by train or bus.