What is the summary of "The Signal-Man" by Charles Dickens?

This chilling ghost story revolves around an encounter
between the narrator of this tale and a railway signalman. During their first meeting
the signalman tells the narrator that he has seen some ghostly apparitions. Actually,
the signalman at first believes that the narrator is another apparition, which explains
his fear of the narrator. He explains that the narrator's greeting was very similar to a
ghost that he has seen by the tunnel's mouth. So strange does the signalman act and
appear that the narrator initially suspects that the signalman himself is a
ghost.

The narrator returns to the dark and gloomy station
house the following evening and the signalman tells him more about the ghost that he
saw. It apparently resembled a man who appeared in front of the tunnel gesticulating
frantically and shouting "Look out!" Although the signalman contacted other stations,
six hours later, a terrible crash happened. Another time, the ghostly visitor returned,
this time appearing to be incredibly sad. Later, a woman died on a train passing by the
signalman. The signalman confesses to the narrator his guilt at not being able to save
any lives from these mysterious apparitions. Although the narrator would like to help,
he cannot think of anything he can practically do to assist
him.

Quite soon after this, when the narrator himself is
out walking, he too sees the same apparition at the mouth of the tunnel, frantically
waving some kind of warning, with its hand covering its face. Hurrying to the
signalman's station, he discovers that the signalman was ran over and died that morning.
A labourer on the railway informs the narrator that the signalman was with his back to
the oncoming train. Identical to the phantom that the narrator saw that evening, the man
covered his eyes so that he wouldn't see the death of the signalman as he continued
gesticulating wildly in warning.