Speaking from experience, sometimes there will also be issues figuring out which seats are actually open when the train is right at capacity. Happened to me one time at ABQ when I was doing award sleeper RVR-ABQ, paid coach ABQ-FLG. I ended up happily plopping in the lounge and the conductor didn't force me to move when they did find the seats.
Amtrak will oversell by a limited amount, but as noted the fact that they have seating in the lounge car or can plop people in the seats in a spare sleeper room helps with this as well.