The accident was caused by an apparent brake fail.
Commuters on board were injured as they tried to disembark the moving train after it failed to stop, the report says.
"At around 19:00 on Friday evening, Netcare 911 paramedics received a call for a train accident in Johannesburg," said Netcare 911 media liaison officer Chris Botha.
Several ambulances and advanced life support paramedics were dispatched to the scene at Ennerdale Station to assist with the injured.
"When paramedics arrived they found that the train did not crash or derail at all. Reports indicate that for reasons still to be investigated the brakes seem to have locked causing people to fall. In the panic it appears that some of the commuters jumped from the moving train."
"After triaging the injured medics found that an estimated 120 people sustained injuries ranging from serious to minor. Paramedics from various services treated the inured at the scene before they transported them to hospitals in the area for further medical care."
Earlier this year, more than 200 people were injured when two trains travelling on the same line crashed at Booysens, south of Johannesburg.
Metrorail faces a challenge of railway infrastructure ageing and not enough maintenance being done.
