What noise does capote hear while the passengers were sleeping on the train?

In Truman Capote's "In Cold Blood," while the passengers are sleeping on the train, he describes a variety of noises, including the rhythmic clattering of the train wheels against the tracks. This sound creates a sense of movement and isolation, emphasizing the stillness of the night and the vulnerability of the sleeping passengers. The noise serves as a backdrop to the unfolding events, highlighting the contrast between their peaceful slumber and the tension building in the story.