I am a bit overwhelmed
Small overview:
I need 9 travel days right?
Where the seats are not available, can I also reserve a seat on the spot and/or travel with the Interrail "for free" without a seat.
If you calculate 9-5 travel days you come to 4
5 = Bucuresti North until back in Bucuresti North.
Would it be worth it because the trains are so cheap there in the east?
Instead of the 10 days within 2 months for 316 euros. 4 days within 1 month for 194 euros. Price difference 122 euros.
Book Zurich - Budapest on tickets.oebb.at : no booking fee, add Interrail/Eurail as a discount and select one-way tickets.
Budapest - Zurich has optional seat reservations : 3€ on tickets.oebb.at (much less expensive than 8€ on interrail.eu).
All night trains should be available at SBB ticket counters (maybe not Bucharest - Halkali, see later) : no fee compared to 2€ per person per train.
You should make some calculations. It's true that tickets are cheap in Eastern Europe. seat61.com will help you a lot. I've given you info for Bucharest - Halkali : better to book a regular ticket online than use a pass day.
Count how many days you need : on a pass day you can board as many trains as you'd like, for night trains as long as you board before midnight only the departure day counts.
Trains in many countries do not require seat reservations (like in Switzerland), simply add the train to the app (like any other train) and board
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