Train is too complicated. Take a Venezia Lines ferry to Pula (Pola): venezialines.com/eng/index.a... And than bus to Opatija.
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I think a better - and cheaper - alternative would be to take the train to Trieste and then the bus to Opatija. The bus station is next door to the train station. Here is the bus schedule: saf.ud.it/orariPercorsi/ts-a...
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