South African logistics group Transnet said on Tuesday it would reopen a coal export line leading to the Richards Bay coal export terminal at 1400 GMT after a derailment over the weekend.

A train derailed on the line near Ermelo in the northeastern Mpumalanga province on January 22. Some 90 export coal trains were cancelled as a result of the three-day line closure, Transnet said in a statement.

It was still unclear what caused the derailment, although heavy rains in the last month have disrupted rail freight operations, affecting both coal and maize exports from Africa's biggest economy.

South Africa is a major exporter of coal to power stations in Europe and Asia, but exporters have failed to ship all their export coal to the Richards Bay Coal Terminal due to bottlenecks on the approaching rail lines.

South Africa exported 63.43 million tonnes of coal last year, boosted by demand from China and India.

Transnet is investing heavily in new and improved infrastructure, but its lines are still too small to meet the terminal's expanded annual capacity of 91 million tonnes.

The heavy rains have also affected mining firms, preventing them loading and off-loading rail wagons. Companies in the coal sector include Anglo American, Xstrata, Optimum Coal and Exxaro.

Low coal stocks at RBCT have led to congestion at the port as vessels wait for days to load.