Question

the thought of laughter only made me feel troubled and lonely​

Answer 1

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The narrator had found the company of the girl very interesting. But, he could enjoy her company only for a short while. The train was approaching Saharanpur and the girl would get off the train at Saharanpur. Once more, the narrator feared, he would be left lonely and alone. The thought made him melancholic. Therefore, he could not laugh.