Abbots Ripton Rail Accident
The Abbots Ripton rail disaster occurred on 21 January 1876 at Abbots Ripton (England), when a passenger train from Edinburgh to London was involved in a collision, during a blizzard, with a coal train. A passenger train travelling in the other direction then ran into the wreckage. The accident totaled 13 deaths and was responsible for fundamental changes to British railway safety practices.
The main cause of the accident was a retrospectively poorly designed semaphore for signaling the trains, where a clear light meant proceed, and a red light means caution. When the semaphore failed because of ice buildup from the blizzard, the failed semaphore’s default clear light was interpreted as a ‘proceed’. This led to adoption of the now ubiquitous red-green signal lights instead of the red-clear.
More information on Wikipedia.