Hi. I'm Physicsgirl, and I'm an idiot. (Hi, Physicsgirl!)
Anyone who has talked to me about this trip will know that I've been relying on
www.tripadvisor.com for many ideas. Almost everyone on those forums said to avoid taking the train overnight from Paris to Venice. The couchettes are very cramped (especially the 6-berth ones) with not enough space for luggage, and there are lots of thieves. I read many stories of people who lost cameras and money.
I was determined, however. I hate flying, I like trains, and it would be travel plus lodging all in one. I'd managed to find a fairly inexpensive 2-person berth, which should alleviate the crowding problem somewhat, and I bought a special travel safe to keep my valuables safe, so I felt pretty good.