The South Croydon rail crash on the British railway system occurred on 24 October 1947.^[2]

The crash took place south of South Croydon railway station. Two electric commuter trains collided in fog and 32 people were killed, including the driver of the second train. It was the worst accident on Britain's Southern Railway.

The crash was a rear-end collision caused by a signalman's error. The inexperienced signalman at Purley Oaks forgot about a train from Haywards Heath to London Bridge standing invisible in the fog. The line was protected by the Sykes "Lock and Block" apparatus, which prevented him from allowing another train into the section until the preceding one had left it. However, he believed that the elderly apparatus was faulty and used a release key. This allowed a train from Tattenham Corner to London Bridge into the same section, and they collided near South Croydon Junction. The trains were crowded in the rush hour, carrying 800 and 1000 people respectively, hence the heavy death toll.^[3][4]

Similar crashes, all caused by misuse of the Sykes release key or similar override mechanism, occurred at Battersea Park in 1937, Barnes in 1955 and Crayford railway station in 1959.