What is the most efficient routing to get from Split to Ljubljana (Slovenia).
It depends on what you mean by efficient
In terms of speed hardly you would bit route Split-Zagreb-Ljubljana, but if you ok making extra changes and spending a bit more time, then it might be interesting to go via Rijeka. To do so you'll need to catch one of the trains Split-Zagreb, but get off earlier in Ogulin, Ostarije or Karlovac (depends on train you take), while from there continue to Rijeka (another 2:30-3 hours journey). From Rijeka there are two direct trains to Ljubljana (dep.11:55 and 20:40, both take a bit less than 3 hours to get to Slovenian capital). For better visualization see Croatia/Slovenian railway network map
here.
With route Split-Ogulin/Ostarije/Karlovac-Rijeka-Ljubljana you basically have few options: a) depart from Split with overnight train, change early morning in Ogulin, arrive to Rijeka 9am, then either catch 11:55 train to Ljubljana or spend the day in Rijeka (it is a nice town with many thing to see) and depart to Ljubljana with 20:40 evening train; b) depart with the morning InterCity (IC) train, change in Karlovac & Ogulin and arrive to Rijeka 18:15, spend 2+ hours in Rijeka and catch 20:40 train to Ljubljana
Alternatively you can also catch an overnight ferry from Split to Rijeka (19:30-7:00, Jadrolinija, runs twice a week dep. Thursdays and Sundays), while then continue by train
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