One of the central concerns of this excellent short story is the way in which characters are shown to be either insiders or outsiders and how such discrete categories can at times overlap. We are presented with a newly-married couple, who are obviously unsure about their status and how they fit in. In particular, as they are on the train, the married couple believe that the opulent and grand surroundings on the train mirror the "glory of their marriage." However, note how the black porter responds to this and how he treats them:
...the man's face in particular beamed with an elation that made him appear ridiculous to the black porter. This individual at times surveyed them from afar with an amused and superior grin. On other occasions he bullied them with a skill in ways that did not make it exactly plain to them that they were being bullied. He subtlyused all the manners of the most unconquerable kind of snobbery. He oppressed them; but of this oppression they had small knowledge, and they speedily forgot that infrequently a number of travellers covered them with stares of derisive enjoyment.
The black porter is thus a symbol of a social outsider, but who is able to take advantages of the insecurities of the married couple to act as an insider for one moment due to the change of status of the married couple and the way in which they are unsure of themselves and their social status. It would be an interesting exercise for you to look at the characters symbolically and see how they relate to these two categories of outsider and insider to analyse what such labels suggest about the characters and the larger symbolic significance of belonging to the society that this story presents us with.