History: Great Smog Of '52, London, England, United Kingdom

Near railway lines, on which 'fog working' was implemented, loud explosions similar to the report of a shotgun were a common feature. These explosions were made by 'detonators', a form of large percussion cap placed on the track and activated by the wheels of trains. These were placed by certain signals to provide an audible warning to match the visual indication provided by the signal for the driver.
Health effects
There was no panic, as London was renowned for its fog. In the weeks that ensued, however, statistics compiled by medical services found that the fog had killed 4,000 people. Most of the victims were very young or elderly, or had pre-existing respiratory problems. In February 1953, Lieutenant-Colonel Lipton suggested in the House of Commons that the fog had caused 6,000 deaths and that 25,000 more people had claimed sickness benefits in London during that period.