The narrator describes learning about his brother's arrest from the newspaper that he reads on his way to work. Based on the context of the story, the reader can infer that the narrator uses the subway as a means of transportation on a daily basis, making it a part of his normal routine.
On this particular subway ride, however, the narrator experiences a disruption in his routine. Routines give us a sense of order and stability, and while we don't tend to think about our routines as comforting, they are. When one's routine is disrupted, either by some inconvenience or stress, one is also mentally disrupted.
This is the case for Sonny's brother in the story. During his daily ritual of reading the newspaper on a darkened subway car while going to work, the narrator is confronted with the existence of his troubled brother. In the paragraphs that follow, the narrator describes "a great block of ice settled in my belly and kept melting there all day long," a feeling that stems from Sonny becoming "real to [him] again." This shows that the narrator's experience on the subway disrupts the rest of the day and maybe even his life.