We did this a few years back, and very much enjoyed it. The highlight was a scenic train ride in the Swiss Alps... absolutely breathtaking! Switzerland was lovely altogether, but I can't remember the name of the places. It was very expensive and there was a lake and a lindt factory.
If I could do it again I'd spend more on accommodation - we used mostly hostels and very cheap options, thinking it wouldn't matter as we'd only be there to sleep - and it really wasn't great. I'd also prioritise having at least one full day and night in each city, because I was beyond exhausted. I can't remember exactly where we went, but Amsterdam, Germany, Belgium, Spain, France, Switzerland and Italy, from what I remember. In about 9 days, I believe.
We did sleep in one train, which was a couchette I believe, where the seats fold up and the beds fold down. I think they sleep 6 people. In the same compartment was a lady and her daughter who were lovely, but I think I was overoptimistic at what the quality of my sleep would be like in a moving train with strangers in the room. I'd try an overnight train again, but only with a private room.
In any case, don't forget your headphones and earplugs! The sound of the train gets very wearing after a while. Also don't forget to write down the journey stages in advance of the conductor coming around. I forgot and was accused of fraud at one point, though luckily he was laughing!