The Signalman- (short supernatural story)- 1976

The Signalman is a short film which is part of the British supernatural anthology series A Ghost Story for Christmas. first aired on BBC1 on 22 December 1976,
It stars Denholm Elliott as a lone railway signalman on a remote country line. Who is visited by a traveller (Bernard Lloyd). The signalman reveals to his new friend that he is being haunted by a spectre, which has appeared by the red warning light, at the entrance, of the nearby tunnel.Also that he hears the warning bell ring,in the signal box but the traveller does not hear it.The traveller tells the signalman to concentrate on his job, and all will be well.
Is the spectre real or just imagination ?. It seems to warn of impending disaster, but if it is real, what is the danger,and when will it happen ?