What is the main reason of train wihich runs without engine

Ahmedabad puri express train moves on track without engine. The coaches of the train started rolling down the track from Titlogarh to ksingha JN .
All passengers were safe and two employees are suspended shortly after the incident came to light. The accident is took place because manually brakes are not applied.
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Видео What is the main reason of train wihich runs without engine канала Train Gyan