One of the reasons that Pirandello's story is so effective is that it rests upon the fat man's certainty.  He enters the car and commands the center of attention.  He gives a spirited discussion as to why he is glad his son chose the path he did.  There is little weakness in his argument.  He delivers it with so much bluster and so much zeal that it is something that cannot even be questioned.  There is no hesitation or reticence in what he is saying. His entry and his declaration are seen as antidotes to the ambiguity that the other passengers are immersed in prior to his entry.  At the same time, Pirandello makes it clear that the certainty of the fat man is precisely what compels the bundled up woman to speak.  She does not say anything.  In fact, Pirandello describes her as "waking up."  She does this because she senses the fat man's certainty and his sense of absolutist faith in what he is saying and in what he believes about his child's death.  She finds encouragement from him because of his own sense of confidence.  This would clearly suggest that the fat man was certain of what he was saying. He is only disarmed when the woman speaks, asking her fatal question that reveals his own certainty as nothing more than illusion.