Their website says you can book e-tickets using the mobile app:
https://potniski.sz.si/en/online-and-mobile-ticket-purchase/
But they don't actually provide a link to the app from that page, or any other page that I can see (wtf!).
I've tried searching the App Store and googling, but I can't find the app anywhere.
Slovenian national railway operator Slovenske železnice (SŽ) has launched a mobile application for ticket purchase. The programme is available for installation on devices using both Android and iOS operating systems
Did you search for the app using -- Slovenske železnice (SŽ) -- ?
If the iPhone App Store is searched for using Slovenske zeleznice (Sz) (without accents because I don't have those characters on my iPhone) then it returns no results.
If Slovenske zeleznice is searched for then there's nothing that looks like its the app in question.
Did you search the App Store and find it?
