South Africa's state-owned freight transport firm Transnet plans to build a $2 billion railway through Swaziland to create a new link to Indian Ocean ports, the company said Thursday.

"This new line will create additional capacity of 15 million tonnes, which will predominantly be general freight volumes from the existing coal export line," the company said in a statement.

The line will open up a new route to the South African port at Richards Bay as well as Mozambican ports.

The 146-kilometre (90-mile) line will cut across Swaziland, and is expected to boost the coal export capacity between the three countries and provide an "unprecedented economic boost for this region", it said.

"Significantly, this is the first large-scale rail investment in Southern Africa since the construction of the Richards Bay line in 1976," said Transnet.

The port of Richards Bay houses the Richards Bay Coal Terminal, the largest in the world. Freight rail is Transnet's largest division.

Construction is expected to take place between 2013 and 2016, with the line fully operational by 2021.

"We expect the first train to run in three years time, after the necessary land purchase agreements and environmental approvals have been resolved," the company said.