The woman who was separated from her son as a result of the war sits in the corner and asks the question that drives the stake into the heart of all the rationales and justifications behind war. I think that her question of "Then... is your son really dead?" helps to bring to light the idea that while others can explain and make the case for war, parents and loved ones have to end up shouldering the responsibility for it. In this light, what ends up happening is that nothing can explain nor take away from the pain of loss that war causes. Even if one wants to use the "fat man's" explanation of "the ultimate sacrifice," the woman's question still has to be reckoned with and still must be accepted as fact. The pain of loss that comes with war is one that can never be fully healed, and never be fully explained away.
To me, her question, and the answer it gets, show how deeply war affects people.
The fat man has been talking so much about how glorious war is. He has been saying that his son was proud to die for his country. He seems to be at peace with the fact that the war has taken his son.
But then the woman asks her question and he falls apart. It is as if he had not really been understanding that his son was dead. When she asked the queston, the truth was forced upon him and he was no longer able to keep up his pretense.
So her question forces us to think about what war really means -- it really means that people we love die.
The woman who has been holding herself away from the other passengers becomes the one to raise the vital question with the heavyset passenger. A possible answer is that she really does not consider the abstract issue of war, but is genuinely concerned only about the inevitable loss of actual life. When she raises her question, therefore, she does so out of her realization about war and death. There are no abstract questions about love for the fatherland for her, or for the need of sacrifice. All she is concerned with is the straightforward truth that war kills people.