We want to go from Varenna to Bolzano/Castelrotto, Italy by train. Is it possible to do that in one day? How difficult would it be to then proceed from Bolzano through Austria by train and arrive in Venice? Is Bolzano to Venice via some northern route possible in one day? Should we buy Eurorail passes in advance for these?
If that's all you are doing, Como to the Alto Adige and Venice, a rail pass probably would not pay. There is a rail line that goes from Milan to Venice through Verona, and at Verona it meets the rail line that comes over Brenner Pass from Innsbruck and down to Verona via Bolzano. The fastest route from Verenna to Bolzano (5-6 hrs) would be via Milan and Verona. Going north, from Varenna through Switzerland (Chur) to the Austrian Tirol and to Innsbruck, then over Brenner Pass to Bolzano might be a more scenic route, but rather circuitous due to the Swiss mountains, would require a lot of connections, including a bus for the fastest route, and take around 11 hours.