We're starting our trip in Slovenia and won't be needing a car there, I hope. Is it possible to take a train from Ljublijana to Rijeka and then rent a car for Croatian travel? It appears so from the map but I'm not seeing anything definitive in the book at this point.
You can find the schedules on the German Rail website:
http://www.bahn.com/i/view/overseas/en/index.shtml
Absolutely possible. I did it in reverse in 2008. However, consider Opatija instead of Rijeka. Opatija is much prettier and has a small train station just a few minutes above the town (Opatija-Matulji). Several rental car companies have an office in Opatija. We had excellent service from Sixt car rental in Croatia. The train that runs that route is pleasant and has that "rickety" regional train feel. Advance reservations are not necessary. Passport control is quick and a breeze in Ilirska Bistrica (no need to leave your seat).