They realize they are not staying in Hungary. They are in Kaschau.
When Wiesel says that their eyes were opened, he means that they are aware that the Nazis mean them harm. They are no longer in Hungary. Specifically, the train was in “Kaschau, a small town on the Czechoslo-vakian” (Ch. 2). A German officer enters their train car and talks to them through an interpreter.
"From this moment on, you are under the authority of the German Army. Anyone who still owns gold, silver, or watches must hand them over now. Anyone who will be found to have kept any of these will be shot on the spot. …” (Ch. 2)
It is easy to have hope until you are actually herded onto a train car and told by a Nazi that you are under the authority of the German government and it’s time to hand over your gold. At that point, you know that you are in trouble.
Remember that we have the benefit of hindsight. We know about the Holocaust because we have always known. It must have seemed impossible then. They did not actually know where they were going or what was going to happen. No one was really telling them anything.
This is the reason that Mrs. Schachter “lost her mind” (Ch. 2). From then on, an impossible situation got even worse. Wiesel says that they were left locked in the train car, and there were eighty of them.
The two disappeared. The doors clanked shut. We had fallen into the trap, up to our necks. The doors were nailed, the way back irrevocably cut off. The world had become a hermetically sealed cattle car. (Ch. 2)
Mrs. Schachter seems to have a premonition. She begins to scream about fire. Of course, we all know what is about to happen to the Jews when they reach the concentration camps, but there is no way that she could have known about the gas chambers or ovens.
The incident with Mrs. Schachter is a remarkable incident of foreshadowing. Yes, it is also a tragic incident of a woman whose psyche could not handle the horrible abuse inflicted on her by her captors, but it is also remarkable for its foresightedness. She seems to have known what was going to happen.