Is "The Signal-Man a ghost story or is the ghost only part of the signalman's imagination?

I do not know that there is any way to answer this question for sure.  I tend to believe, however, that the ghosts (at least in the story) are actually real.

The main reason that I think the ghosts are "real" is that it is not only the signal man who sees them.  He is the one who sees most of them but, at the end of the story, the narrator sees one of them as well.  This implies that they must be real.

The other thing that makes me say they are real is that they appear to predict bad things that are going to happen.  It does not seem likely that the signal man could imagine such things in advance of the bad things actually happening.