The narrator describes the Signal-man's box as supernaturally eerie and desolate:

So little sunlight ever found its way to this spot, that it had an earthy deadly smell; and so much cold wind rushed through it, that it struck chill to me, as if I had left the natural world.

The “deadly smell” and “chill” enhance the unnatural atmosphere, and the box’s location at the bottom of a deep ravine evokes an almost hellish landscape, punctuated by the red danger light. This description lends a suspenseful and dread-laden atmosphere to what otherwise may have seemed like a normal conversation between a traveler and a railway signaler.

The monstrous thought came into my mind as I perused the fixed eyes and the saturnine face, that this was a spirit, not a man. I have speculated since, whether there may have been infection in his mind.

The Signal-man appears like an apparition, or ghost, to the narrator. His “saturnine” visage projects gloom, and his eyes are constantly seeking out specters that only he can see. He is a fitting resident for the eerie and unnatural landscape he occupies. His odd behavior leads the narrator to wonder if he might have an “infection in his mind,” a common nineteenth-century euphemism for madness.

He had been, when young (if I could believe it, sitting in that hut; he scarcely could), a student of natural philosophy, and had attended lectures; but he had run wild, misused his opportunities, gone down, and never risen again. He had no complaint to offer about that. He had made his bed and he lay upon it. It was far too late to make another.

The Signal-man is portrayed as having fallen in station to the point that he now resides in an environment that resembles hell. His isolation and the spectres that appear only to him can be read as a commentary on the increasingly unnatural effects of technology on human nature. The fact that it is “far too late” lends a sense of fatalism to both the Signal-man and the story as a whole, foreshadowing the grim ending.

“What is its warning against?” he said, ruminating, with his eyes on the fire, and only by times turning them on me. “What is the danger? Where is the danger? There is danger overhanging somewhere on the Line. Some dreadful calamity will happen. It is not to be doubted this third time, after what has gone before. But surely this is a cruel haunting of me. What can I do?”

The Signal-man has come to associate the apparition with death and disaster. However, his inability to prevent the incidents that the specter portends vexes and dismays him. The Signal-man seems less haunted by the apparition itself than he is by guilt and anxiety about his own apparent helplessness. The train and its accoutrements are a looming presence in his life, a source of constant anxiety, suggesting that perhaps the apparition is a manifestation of the broader social anxieties of the postindustrial world.

Without prolonging the narrative to dwell on any one of its curious circumstances more than on any other, I may, in closing it, point out the coincidence that the warning of the Engine-Driver included, not only the words which the unfortunate Signalman had repeated to me as haunting him, but also the words which I myself—not he—had attached, and that only in my own mind, to the gesticulation he had imitated.

In the end, the narrator realizes that he has seen exactly what the Signal-man saw as an apparition—a man with his left sleeve across his face, waving his right hand. The man also repeats the exact words that the Signal-man had heard before from the apparition, with the addition of a phrase that the narrator thought but never spoke aloud. The narrator is left chilled by the revelation that both he and the Signal-man appear to have played a part in foreshadowing the “curious circumstances” that befell them.