The tunnel represents people's fear as it is dark and foreboding--"a great dungeon." Also, the tunnel is crooked so that one cannot see through it to the end, and the entrance is dark and gloomy with a "barbarous, depressing, and forbidding air."
Certainly, there is something of the Gothic about this dismal tunnel that is placed in an isolated, "lonesome post" and is so shaded from the sun that it has an "earthy, deadly smell." Such a dark, dismal, and damp channel as this lone tunnel easily can prompt a person's fears of the unknown; additionally, the signalman himself is a rather daunting figure to the narrator:
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.
Such a shadowed, lonesome, and damp place as this tunnel and a signalman who has an "inexplicable air upon him" truly generate a representation of people's fears of the mysterious and the threatening unknown which can profoundly disturb the isolated imagination.