Hello,
about your 1st and 2nd question:
- Yes, it rides on the same tracks like the Bernina Express but with other train sets.
- there is no direct service from Tirano to Venice. You take regional express trains from Tirano to Milan and from there you can take Frecciarossa high speed trains to Venice (compulsory reservations). You can take regional express trains on the same route, but they need longer and you have to change in Verona trains.
Some advice:
If you're a British citizen living in the UK you need an Interrail Pass. The country is still part of Europe, same as Switzerland, Norway or Iceland!
About the logical order: the Bernina railway is definitely stunning so good choice!
From Venice you can reach Budapest in a long 11h travel day (via Vienna). But as you said, also possible to take the Venice - Vienna night train (using shiny new carriages!), spending some time there and travelling to Budapest later that afternoon.
About pass days (00:00-23:59): the train's departure time/date is what matters. On night trains only one pass day is required then. For Vienna - Budapest either you use another pass day or you buy a cheap advance ticket instead.
Yes, you should activate the pass on the day you're starting your trip. However some mandatory reservations might need advance booking: night trains, Eurostar, infrequent TGVs, etc. In your case book the night train ahead if you plan on taking one. Milan - Venice (13€) can be booked at short notice: hourly trains, plenty of seats.
Feel free to ask anything. The Man in Seat 61 website is an incredible resource. :)
It sounds like a lot of travel for a week. Up to Tirano is fine. But then trying to get to Venice and Budapest might be stretching it. Venice is a bit of a dead-end.
- The regional trains (and the unreserved coaches on the Bernina Express) are great. Windows are big anyway. Absolutely no need to pay for a reservation
- As above, you’ll need to change in Milan. Venice is a bit of a dead end based on where you’re going. Sleeper to Vienna seems the obvious way out.
- I think your itinerary is already packed, but yes you can stop in Trieste if you want to
- Yep, don’t activate your pass until you’re definitely travelling, just in case plans change. Same with each new travel day.
- I’ve stayed in a few hotels in Tirano and Chur - none were particularly cheap or memorable! You don’t give an idea of budget, but in Venice I stayed at the Meninger Hostel in Mestre (private rooms too). It’s literally on the platform of Mestre station, so very easy to commute into Venice. Brand new building too. If you stay in Vienna, I really liked the Social Hub near Praterstern station - you can get the S-Bahn there with your pass.
- Google Flights allows you to search on the map for the cheapest flights. With a railpass, you can reach anywhere. Look for airports that have good rail connections - I can think Trieste, Munich, Vienna, for example. Loads more.
But if you’re in Budapest, why not continue all the way by train? It’s only a hop skip & jump: one sleeper to Bucharest, and another to Istanbul.