The accident, which took place on the line near Ermelo in the northeastern province of Mpumalanga, shut two lines from the mines to the export terminal.
RBCT said that exports would not be affected as the terminal had enough stockpiles for now.
"From the terminal's point of view, there is no impact, we have adequate stocks at the moment," said RBCT chief executive officer Raymond Chirwa.
But the significance of another derailment is not its impact on immediate shipments but the further doubt it casts on the country's ability to export its targetted 65 million tonnes this year, exporters said.
"Transnet have had a good performance for the past few weeks but just when you start to think things are OK, something else happens," one exporter said.
"So 65 million tonnes is looking rather unlikely and we haven't even got to the maintenance period or scheduled pay talks which could lead to another strike," he added.
There has been a series of derailments between December and February which cut over 3 million tonnes from exportable coal, exporters said.
South Africa's freight logistics group Transnet [TRAN.UL] said on Friday that it had reopened one of the two lines linking coal mines with the Richards Bay Coal Terminal (RBCT) but could not confirm when the other would reopen.
"The first line reopened this morning. We will inform you as soon as there is some indication about the second line," spokesman Sandile Simelane told Reuters.
The lines near Ermelo are particularly prone to derailments.
Besides the immediate impact, the derailment also means trains have to be rerouted, causing further disruption to the transport of coal. An investigation into the cause of the derailment was ongoing, Transnet said.
South Africa is a major exporter of coal to power stations in Europe and Asia, but exporters have failed to ship all of their product because of bottlenecks on the lines approaching the huge Richards Bay Coal Terminal.
South Africa exported 63.43 million tonnes of coal last year, boosted by demand from China and India, but far below the terminal's expanded capacity of 91 million tonnes.
Transnet is investing heavily in new and improved infrastructure, but it will take years before a substantial increase in transported tonnages is seen. (Reuters)
