A runaway diesel locomotive smashed head-on into a freight train and burst into flames, killing one train driver and critically injuring another, a trade union said on Friday.

Train drivers Alexander Flemming and Jan Swanepoel, and train assistant Neo Jason Witkoei, were on their way from Rosmead in the Eastern Cape, to Kimberley in the Northern Cape, on Thursday, United National Transport Union (Untu) general secretary Steve Harris said.

They had been waiting for about five minutes at a signal for the go-ahead from the train controller. Suddenly they saw the locomotive bearing down on them at high speed from the Noupoort direction.

Witkoei managed to jump from the train, before the diesel locomotive exploded on impact.

Harris said Swanepoel was found lying in the grass next to the train, covered in oil from the explosion.

Witkoei and people living nearby dragged Swanepoel from the flames and waited for an ambulance to arrive. Swanepoel was taken to hospital and was still in a critical, but stable, condition on Friday.

"Flemming could not manage to escape the locomotive in time," said Harris.

“It is every train driver’s biggest nightmare to be confronted with a runaway locomotive. This is a freak accident and it does not happen often. Transnet will now conduct an investigation and a board of inquiry will establish what happened and if anyone can be held accountable."

A Transnet spokesperson was not immediately available to comment.

Harris said that the diesel locomotive did not appear to have any occupants, as it had been staged at a nearby station.

The investigators would have to find out why it was able to move downhill at such high speed, considering a staged train was supposed to have blocks against its wheels to prevent it from moving.

An added complication was that the communication system was off-line, so the controllers would not have been able to detect unplanned movement on the track to warn the three.