Thanks for that, it looks like we'll be staying at Pula. I mentioned Rijeka because it looked like more of a transport hub.
I've looked up the caves, will try and see them while we're there.
I'd say the Roman remains in Pula are quite spectacular.
A lot of the Adriatic coast line (in Croatia, Slovenia and further south) has a sudden, spectacular drop from the karst hinterland to sea level, making for long, tortuous zig-zag rail routes for any lines that connect to the sea ports/resorts, including Rijeka (which is electrified, IIRC).
I think you mean Postojna (not Postonja)? Not trying to be picky - just if you make a mistake like this you can end up with the wrong ticket to the wrong place. That line (from Ljubljana to Trieste) is also quite slow and tortuous - which makes it v scenic, of course.
Yes, Postojna caves are spectacular. If you go on a rainy day in summer, the tours tend to be quite full. Bit of a trek from Postojna station to the caves.
ps I'd say Croatia, at least on the coast, is not cheap these days. Unfortunately there tends to be a bit of a 'We only have three months, let's screw the tourist while they're here mentality".