I was in Manchester Piccadilly today and whilst awaiting my train to Doncaster I observed the departure of a train on the opposite platform. The Guard of the train was awaiting a green/amber signal and I noticed how far away the signal was at the top end of the platform. This got me wondering if this signal would actually be visible during thick fog and what would the procedure be if the signal was not visible? I assume the signals must have some sort of special lighting as I don't ever recall the rail network grinding to a halt during thick fog! Many thanks.