Two workers, one of whom was pregnant, were killed when a luxury Rovos Rail train derailed near Pretoria on Wednesday, a spokesperson for Netcare 911 said.

Chris Botha said the pregnant woman went into labour immediately after the accident, possibly from the force of the impact, but neither she nor her baby survived.

"We don't know how far advanced her pregnancy was," Botha said.

"But the foetus was very small. They both died."

Earlier, ER24 spokesperson Werner Vermaak said five passengers were critically injured, nine sustained serious injuries and 35 minor injuries.

Vermaak said one of the critically injured was airlifted to the Eugene Marais hospital in Pretoria. Other injured passengers were taken to at least three hospitals in and around Pretoria.

"It is a chaotic scene," Vermaak said.

"Patients were strewn all over the scene. We had to dispatch ambulances from Johannesburg as well to cope with the number of injured."

Vermaak said there were 17 ambulances and 10 rescue vehicles on the scene. Some of the passengers had to be cut free of the mangled wreckage of the train.

"Some of the patients were thrown from the wreckage and some were trapped inside," he said.

"The Tshwane fire department had to use hydraulic tools to free the trapped passengers from the wreckage. Most of the injured sustained fractures and full body injures when the train derailed."

Rovos Rail's marketing manager, David Patrick, said the cause of the derailment was not yet known, but would be investigated.

"We don't know much yet," Patrick said.

"We had an incoming train from Cape Town to Pretoria, and it derailed between Centurion and the Pretoria (railway) station."

Patrick said the company was part of the National Rail Network and an independent investigation would be launched in conjunction with Spoornet.

The Railway Safety Regulator said it had learnt of the derailment with "alarm".

Spokesperson Lawrence Venkile said the accident affected all commuter rail services on the Pretoria main line. The affected portion of the line had been closed until further notice.

"Our Railway Safety Inspectors are at the scene of the accident to immediately conduct an investigation on the root cause of the accident," Venkile said in a statement.

He said a preliminary investigation showed the train was on its way to Capital Park, outside Pretoria, en route from Cape Town.

It stopped in Centurion to change locomotives. Before the new locomotive could be coupled, the rest of the train "went into motion and ran away... resulting in a derailment outside the Blue Train depot in Pretoria".

Venkile said the investigation would focus on procedures followed in the changeover of the locomotives, the condition of the brakes of the train, as well as "other factors which may have contributed to the accident".

Rovos Rail was established in 1989 and offered "unique train safaris through the heart of Africa", according to its website. The company runs two trains with a passenger capacity of 72 each.