The mystery of the signalman's death lies in the character's diligence and his sensitivity as a "student of natural philosophy." In this story, Dickens employs details that stress the signalman's careful attention to his duty, his faithful adherence to routine, and his constant vigilance. After meeting the narrator, the signalman asks him to come to visit one night so that he can inform his guest of all the details so that together they can, perhaps, deduce how to solve the mystery of the voice that is heard.
Overriding all of these characteristics of the signalman, however, is his extreme isolation and loneliness. In fact, with no names given for either the signalman or the narrator, the characters are mitigated by the detailed descriptions of the area around the train tunnel with its "crooked prolongation" and "dripping-wet wall of jagged stone" as well as the details of the signalman's office with its fire, official entry book, a telegraphic instrument with its dial, face, and needles, and a little bell.
And, thus, the Victorian conflict between the new technology, represented by the trains, and man takes place. For, there is something foreboding and sinister about the impervious machine that travels through the dark tunnel. In a sense, the train seems an adversary to the sensitive man. With the train's having disturbed nature with the carving of the tunnel as well as by the intrusion of the looming black machine, supernatural forces are set in motion, forces too strong for the signalman to overcome as they seek what may be retribution.
Yet, somehow the preternatural world interferes, making the train impervious to the warnings.