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According to Jeff Stanton's Coney Island History Articles, as posted to ConeyIsland.com
"The Thunderbolt had two serious accidents during the first two years it was opened. A women was killed on August 21, 1925, when she was thrown forward and her head hit the metal handle bar in front of her. A more serious accident occurred on July 26, 1926, when a three-car train stalled part way up a hill. As it rolled back down to the bottom, it was struck by the following train. Twelve people were injured, one seriously."