In "The Signal-Man," the narrator, by nature solitary, feels drawn to interact with a signalman. The signalman, standing by a lonely box near the railway tracks deep in a dark gulley, has the job of waving red flags, operating lights, and sometimes even pulling a metal lever to warn trains of danger or stop them before a disaster can occur. The narrator emphasizes the isolation and forlornness of this man's job.
As they talk, the narrator learns that the signalman has become increasingly anxious and upset because he has been seeing a "spectre" that forewarns of dangers that he is unable to stop, primarily because he doesn't know the specifics of the cases.
While it might be easy to dismiss the signalman's reports of the appearance of the ghost as a figment of his imagination brought on by working all by himself in a depressing place, the signalman has witnessed too many coincidental tragedies since the appearance of the ghost for the narrator to dismiss the signalman's words. Finally, the signalman keeps hearing, from the ghost, the words
Below there! Look out! Look out! For God's sake clear the way!
These are exactly the words spoken to the signalman before he himself is killed, a chilling fact the narrator absorbs.
The supernatural functions to underscore the anxieties felt by people of little power, cut off from human contact, who were forced to carry great responsibilities as the age of mechanization accelerated, responsibilities perhaps greater than one isolated person should have had to assume. As the signalman tells the narrator:
Why not tell me where that accident was to happen—if it must happen? Why not tell me how it could be averted—if it could have been averted? When on its second coming it hid its face, why not tell me instead: "She is going to die. Let them keep her at home"? If it came, on those two occasions, only to show me that its warnings were true, and so to prepare me for the third, why not warn me plainly now? And I, Lord help me! A mere poor signalman on this solitary station! Why not go to somebody with credit to be believed, and power to act!
The ghost underscores the signalman's powerlessness over forces he can't adequately control.