The problem with the quiet car is that offenders usually do not realize they are in it. Unless you travel on Amtrak frequently (I don't do it often but certainly more than the average person) you may not even realize a quiet car even exists. It isn't common to have a quiet section on an airplane or any other form of public transportation so I can see why people would not expect this.

Ignorance is no excuse for being loud repeatedly after this is pointed out but I almost always travel in the quiet car and have found that conductors only infrequently make announcements explaining that the car has special rules-perhaps 25% of the trips I've taken.

I cannot blame someone for missing the sign that says, "Quiet Car" but I can place some responsibility on Amtrak for not making the quiet car and the rules that go along with it more obvious.