Collision on roller coaster in Shenzhen, China: roller coaster suddenly retreats while going uphill = Chinese report

On the 27th, an accident occurred at the Shenzhen Happy Valley theme park in Shenzhen, Guangdong Province, China, in which two roller coaster vehicles collided with each other.
Around 6:27 p.m. on the 27th (local time), 22 passengers were riding on one of the roller coasters called "Yukichi Yutaka" and the vehicle broke down while going up the hill.
The vehicle then backed up and slid down the hill, colliding with another vehicle parked behind it. Passengers were getting off the parked vehicle. The parked vehicle was carrying 24 people and was at the time of the accident.
At that time, 15 people had already completely disembarked, and nine were in the process of disembarking. Immediately after the accident, eight injured passengers were rushed to the hospital. Some passengers subsequently visited the hospital themselves, totaling 28 passengers.
It is known that the customer has been examined. Eleven of them have returned home after being tested. Currently, 17 people are hospitalized and receiving treatment. Four of them suffered serious injuries such as traumatic brain injuries and pelvic fractures.
He is currently in the ICU (intensive care unit), but his condition is said to be stable. The authorities of Nanshan District, where “Shenzhen Happy Valley” is located, are responsible for the emergency management bureau and market management supervision bureau.
An investigation team has been assembled to investigate the cause of the accident.