Can you buy the ticket on board the trains from the conductor? If yes, how much is the penalty?
It is possible to purchase train ticket on board, but you will be charged extra 2,5 euro (obviously only if you "board" the train on the station with the working ticket counter)
SZ doesn't sell e-tickets yet, does it?
Well, Slovenian Railways actually launched
e-ticket sale, but at the moment it seems to sell only weekly, monthly and yearly passes (so-called abonentske vozovnice) and obviously it's only in Slovenian as of now. Perhaps things will change and with time SZ will be selling all kinds of tickets online, but have no idea when it will happen.
I'm planning to travel in summer by the Istra from Budapest to Koper, but it looks much cheaper to buy the ticket only to Ljubljana and buy a domestic ticket from Ljubljana to Koper.
Well, you should keep in mind that discounted sparschiene tickets (starting from 19 euro) are offered only for 'Citadella' daytime train, so it won't be possible to travel with 'Istria' for as low as 38 euro return. While Ljubljana-Spezial for 49 euro return (which should be ok to use for 'Istria') is just 2.7 euro cheaper than 'standard' ticket, 51.7 or 72.7 euro for Budapest-Koper-Budapest according to MAV.
The problem is I'd like to continue on the same train, but it stops only for few minutes in Ljubljana and no time to buy tickets.
Within Slovenia 'Istria' has a long stop in Maribor, but probably at the time of this stop the ticket counter won't be working
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