Why does "Sonny's Blues" by James Baldwin start in a subway? What could the subway symbolize?

Baldwin's "Sonny's Blues" opens in the subway as a way of setting a particular kind of scene, and it also acts as a symbol.  A subway is generally an urban form of transportation, and this tells the reader that the narrator is on a crowded subway in a city.  There is a certain kind of anonymity in being on a subway, I think, and I have always found it interesting that the narrator is in that state of anonymity as the story begins, and that the narrator remains anonymous--that is nameless--throughout the entire story. A subway runs beneath the ground, in the dark, and that darkness is a symbol of the darkness that runs through the lives of these characters, as African-Americans, dark in skin, as people suffering from poverty and prejudice, dark in their lives, and of course, the darkness of the blues that Sonny plays and suffers from in his drug addiction.  The subway also represents the trapped quality of the characters' lives, as the narrator describes himself as "trapped in the darkness that roared outside" (Baldwin 1), as good a way as I've ever seen to set the tone of a story.