Question

what is the ironical situation in which we find miss fairchild and the other two passengers in the train​

Answer 1

The situation is ironical because of fixed perceptions. When Miss Fairchild sees Mr. Easton and the marshall, in handcuffs, she readily believes that the shabbily dressed man is a prisoner being taken to jail by Mr. Easton, who is pretending to be a lawkeeper. For her, the real marshall is a lowly criminal. The situation and the real marshall's goodness of heart brings about a complete reversal of roles.

The irony here is based on appearances. Just because the actual marshall is not smart, he is taken to be the prisoner. Miss Fairchild's name also adds to the irony- she is not fair in her judgement. Her perception is based only on appearances.