Years ago when I was traveling on a Eurailpass, I seem to recall overnight trains holding my passport for the overnight portion of the trip. This was so that border control agents would not have to wake everybody up to check passports every time we entered a new country. Of course, this was a bit different in that my passport was always in the same (relative) location as my person.

To answer the OP's question, I would be highly reluctant to travel anywhere abroad without having my passport on my person.