I was in Europe in 2002, and this question has haunted me ever since. :-)
I made a last-minute booking of a train from Amsterdam to Copenhagen, by way of Maastricht, Liege-Guillemins, and Hamburg. (Crummy route, but that's the best I could get.)
The leg from Liege-Guillemins to Hamburg lasted from 00:52 until 06:52. I had a 2nd-class ticket, and there were no sleepers or couchettes available for rental. When I got on the train, I found that the 2nd class cars consisted of 4 or so compartments per car (each compartment had a non-locking door and a pair of benches that faced each other, so each compartment could fit 6 passengers).
To my dismay, most of these compartments were rather full of sleeping travelers. Some sleeping 'vertically', some lying across more than one seat. Here's my etiquette question: Is it polite/Is it expected that I would 'barge in' on a compartment like this, at 01:00? Is it polite/Is it expected that I would wake up a sleeping person and ask them to sit up so that I could have the seat next to them?
(I lucked out and found a compartment with only two people in it, including a friendly Canadian who waved me in, but I don't know what I would have done had I not!)