what was the author's purpose in adding the flashback about the train ride?

The author uses the train journey to help us explore the character of Esperanza in greater detail. After several days, the train ride comes to an end. Esperanza, Hortensia, Alfonso, Miguel, and Mama arrive at the Mexican-American border. Esperanza and Mama get in line at the immigration office. Esperanza notices that the first class passengers get preference and speed through the process. Esperanza is nervous when she sees that some people are getting rejected from the border, but she and Mama pass through immigration without any complications. They board another train to the United States. Hortensia, Alfonso, and Miguel board at the last minute.