Seat61 definitely helped me set expectations for my trip, Last month I took a Paris-Berlin-Vienna-Venice-Paris loop utilizing the nightjet for 3 overnight trains in a row thinking the cost was at least offset by not needing a hotel each night, but alas, a hotel would have been money well spent as I spent that leg of the trip in a sleep deprived daze !
On the bright side I got the experience of groggily asking a German border guard "what's up?" before I had a handle on why my makeshift curtain was being pulled aside at 3am
German police do check passports on trains within Germany. I was questioned on a train going from Munich to Prague, and as I had overstayed in Schengen, things got a bit tense. Ultimately they decided it wasn't worth it; they got off one stop before the Czech border and let me continue.
I can attest this too; at Aachen Hbf. in April of this year, I witnessed a group of policemen verbally (in English) warning a passenger for travelling across the Belgo-German border without ID.
On a tangential topic, the behaviour of railway staff when checking passports was interesting. With a paper Interrail ticket, they would always ask to see my passport or ID card, yet I don't think I have been asked once when using the electronic Interrail ticket - this is despite the rules being identical for both types.
French Guards board train at Italian border & check everybody immigration (either a EU card or visa and passport), at least in 2016. They take passports with them, saying something in French, and bring it back after about an hour. Longest hour of my life.
So you travelled 11 hours to Berlin in order to spend 9 hours there, then 12 hours to Vienna in order spend 14 hours there, then 11 hours to Venice? You would have been exhausted whatever mode of transport used.
For some reason Americans have a strong tendency to come up with these kinds of itineraries in the name of having a relaxing vacation, even though to any European it appears completely bonkers.
On the bright side I got the experience of groggily asking a German border guard "what's up?" before I had a handle on why my makeshift curtain was being pulled aside at 3am