Day 1. Cologne to Leipzig
In order to travel cheap on this route you have two choices:
a) purchase discounted Dauez Spezial tickets for the whole route (at least one change, from 5 hours on the road). Tickets for these connection start from 29 EUR per person, but in order to get such fare you need to buy them online (
www.bahn.de) really early, like couple month in advance.
b) use daily group pass (called Schönes Wochenende Ticket for the weekend, up to 5 passengers for only 39 EUR or Quer-durchs-Land-Ticket, same thing, but for weekdays, will cost 46 EUR for two passengers). Both can be bought even on the day of travels. But those passes are valid only for regional trains, thus you will need to make extra train changes and spend a bit more time.
Day 3. Leipzig to Dresden
Best choice here is same group pass, but only for Saxony, called Sachsen-Ticket (Saxony-Ticket) cost 29 EUR (same conditions as previous passes)
Day 6. Dresden to Brno
Cheapest option just to purchase discounted tickets from DB website in advance (link was given above), fare start from 19 EUR per person.
Day 9. Brno to Bratislava
Cheaper to purchase your tickets at least 3 days in advance from Czech Railways website (
https://www.cd.cz/eshop/international/search_int.aspx?tt=2), will cost 151 CZK per person, at the station you' pay 234 CZK per person.
Day 11. Bratislava to Keszthely, Lake Balaton
no need to purchase anything in advance here, but better at least to break your tickets via Budapest (anyway you will need to change trains there). Bratislava-Budapest (16 EUR per person), Budapest-Kesztely with Hungarian domestic tariff is 3360 HUF (app. 12 EUR). If your really want to save more, then you will need to add an extra changeover: buy domestic ticket Bratislava-Sturovo (from 6.3 EUR per person), Sturovo is located exactly across the Danube from Hungarian town Esztergom, thus you will need to walk from Sturovo train station to Esztergom trains station (for part of the walk you can take public bus as well), then purchase ticket Esztergom-Keszthely with change in Budapest (4130 HUF or app.15 EUR per person)
Day 15. Keszthely to Zagreb
Simplest way just to purchase ticket already at the station, sounds strange, but return due to special discount will be actually cheaper than one way (app. 16 EUR per person).
Day 17. Zagreb to Belgrade ( overnight sleeper )
Same thing as previous one, just purchase the ticket at the station. Ticket cost around 160 kuna (app. 21 EUR) + sleeper reservation
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