We have ordered our overnight train tics form Vienna to Venice through BETS. We were told it is not unsual to have to show PP's and tics at the Austrian border. Has anyone had this experience? We plan to take Ambien to get some sleep (we reserved a sleeper compartment).
When we went from Prague to Munich, they did check our passport at the border. But we crossed the border probably within 2 hours of leaving (about 10pm), so it was no big deal. If you look at your itinerary (bahn.de probably shows it in detail), you can see what time you'll be crossing into Italy and get an idea. I've also heard of conductors holding on to your passports for this purpose, but we didn't have that experience. I'm sure someone will read this post who has taken the same train ride as you and will know firsthand...
We took and overnight train from Venice to Vienna a few years ago. We had a sleeper compartment and the conductor held our passport until the morning. We were not disturbed during the journey. We then took an overnight train from Vienna to Geneva. Not only did the conductor handle our passports, but she even obtained our stamps for our VAT refunds as well.