CHARLES DICKENS''S LAST GREAT GHOST STORY is also his most personal, inspired by a terrible accident on a train he himself was riding on. He revisited this haunting memory on the figure of a railway signalman, who hears bells ringing in his signal box when no one else does, sees a figure no one else can see... and who, following those ominous signs, always witnesses horrible incidents. ''I am troubled, sir,'' he cries. ''I am troubled!'' But what exactly is it that is troubling the signalman? ... And what does it want?