Two passenger trains packed with school children and rush-hour commuters collided near the South African capital Pretoria on Thursday, injuring up to 300 people in a crash the authorities blamed on cable theft.
The crash took place at around 7.10 am (0510 GMT) when a commuter train heading from the suburbs to the capital ploughed into a stationary train on the same track.
Medical workers said up to 300 people have been treated for various degrees of injury.
"Two are critically injured, one driver and one passenger" and there are 19 seriously injured, said Mosenngwa Mofi, chief executive officer of railway operator PRASA.
It was not immediately known how many children were injured.
"Both of the trains were full of commuters and between them were lots of school children on the way to school," said Johan Pieterse of Tshwane Emergency Services. "We counted about 50 plus children."
Every day around 20 000 people use the blue line between the residential suburb of Kalafong and central Pretoria.
Rescue workers initially struggled to cut away the tangled wreckage of the trains to free the passengers.
One of the train drivers was freed from the carriage where he was trapped for two hours.
"He's critical at this stage," said Pieterse.
Police and railway investigators looking into the cause of the crash zeroed in on the theft of 25 metres (yards) of copper cable linked to the signalling system.
The removal of the cable forced drivers to switch to manual operations, which require a control centre to tell drivers if a section of track is clear before they can proceed.
"What could have led directly to the accident is still subject to investigation," said Mofi. "Cable theft is the root cause of the accident."
While cable theft is common in South Africa, Mofi speculated that the motive may not have been to get the valuable copper.
He said striking railworkers may have been responsible for removing the cable, as part of a pattern of sabotage seen in an industrial dispute with the owners.
"We do have a strong suspicion that it is linked with the current strike," said Mofi. "During the strike there have been serious acts of sabotage."
Transport Minister Ben Martins did not rule out sabotage, but said the police and justice department are investigating.
"It is time to see cable theft as an attempted homicide or attempted murder," he said.
The crash is the latest serious rail accident to hit South Africa's ageing urban rail network.
In 2011, 857 commuters were injured in Johannesburg's Soweto township when a passenger train smashed into a stationary train during the peak rush-hour period.
PRASA has itself described its passengers as "travelling like cattle".
Over 90 percent of commuter trains in South Africa date back to more than 50 years, the most recent dating from 1986.
The network is currently undergoing a major revamp to upgrade its fleet, spending 123-billion rand ($14-billion, 10-billion euros) over 20 years.