A 20-year-old Fukui woman who was riding in the car in question sustained a serious back injury, while a 28-year-old man from Osaka Prefecture who was in a pool near the ride was seriously hurt after one of the roller coaster's metal wheels fell on him [from a height of 8 meters].
Each of the wheels is 32 cm in diameter and 6 cm thick, weighing about 6 kg.
http://www.japantimes.co.jp/cgi-bin/getarticle.pl5?nn20030824a3.htm