How does the ending of "The Signal-Man" come as a shock to the reader?

In Charles Dickens's "The Signal-Man" an unnamed narrator meets a railway signalman who tells of a story in which he, the signalman, frequently encounters a strange specter who serves as an omen of bad fate; each time the signalman encounters the specter, a tragic event occurs. Having seen the specter several times in the recent days without noticing any tragic event, the signalman finds himself in a sense of great foreboding. He wonders what, if anything, will count as the tragic event that the specter foreshadowed.
The narrator, being a logical sort of fellow, holds great skepticism that there is any sort of supernatural correlation between this supposed specter and the tragic events. Yet, when he returns to meet the signalman, he sees a figure on the rails, just as the signalman saw the specter. As it turns out, this figure that the narrator sees is, in fact, the signalman, who is struck by the oncoming train and killed.
The shock of this story comes from the parallel nature of the specter and the signalman, as well as the uncanny resemblance between the identical action performed by both the specter and the driver of the train; they both cover their faces in fear. The shock is made even greater by the thought that the premonition of the specter's appearance would ironically be that of the signalman's death.