Well, there are certainly many different aspects that you could focus on in this excellent short story by Dickens. One of them which I find rather interesting is the symbolism of the railway and how this is linked in with the themes of the story. Essentially the story concerns a phantom that warns or informs a helpless watcher of a fatal accident that is just about to happen or has already happened. In Victorian literature, the railway was a highly ambiguous symbol, as it symbolised fears about technological advances and how they could destroy traditional ways of living, exchanging close and personal connections with more impersonal mechanical procedures. We can see this anxiety in this short story by the way in which tragedies occur in spite of all of the mechanical means of ensuring safety. In spite of signal lights, telegraphs wires and safety lights, the signal-man is haunted by the fact that he is unable to stop the tragedies that occur.
Even though the signal-man is presented as being diligent and hardworking, not shirking in his duty in anyway, it is clear that this is not enough to prevent the tragedies from occurring, and it is this as well that makes him so upset and disturbed. Note how he himself says he used to be a "contended man" but that now he is "troubled" because of these happenings. In spite of the technological advances and mechanical safety systems, the train appears to possess an untamed energy that operates as it will in spite of the systems created by its inventors. Thus this story could be read as a critique on progress and technological advances, and how such progress can be dangerous.