Twenty people had been injured and one train driver was still trapped in the carriage, she told the eNCA television channel.
"At this stage, we do not have any loss of life," said Mofokeng.
The trains were on the same line toward the capital Pretoria when one train rear-ended the other one from behind.
"It is not a head on collision," she told the television station.
The trains were operated by Metrorail, the country's rail system in cities.
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.