Depends on the country, try to travel to or from Romania or Bulgaria next time ;). Not only that they check the ID, but they sometimes check the baggage as well, and when they do that it's a sign that they will find something and cause you trouble unless you discretely pass them 25EUR...

As a Romanian national who has used the train extensively I cannot testify to that. Not once was I asked for an ID and the "25EUR" (usually much less) is for when you don't have a ticket; though I notice the "practice" diminishing.