Johannesburg - Durban considered

allAfrica.com: South Africa: Ndebele Moots High-Speed Joburg-Durban Railway Link

Transport Minister Sibusiso Ndebele is championing the construction of a multibillion-rand, high-speed rail link between Durban and Jo'burg, which would cut transport times from about 12 to three hours.

His department will also be studying a high-speed-rail line between Johannesburg and Cape Town.

High speed Durban–JHB rail link viable, says Prasa's Montana

Passenger Rail Agency of South Africa (Prasa) CEO Tshepo Lucky Montana says that the agency, which houses the Metrorail operations, has completed the prefeasibility study on the proposed venture, adding that the “project is financially viable, but we are convinced it will not work without also carrying freight”.

Montana says Prasa envisions that the train, if it becomes a reality, will travel between 300 km/h and 350 km/h, completing the 600-km-plus distance in 3,5 hours. It currently takes more than ten hours to travel between the cities by train.

He says Prasa is now waiting for the go-ahead from the Department of Transport to move on to a feasibility study.

That looks absurd. Most freight trains go too slow to coexist very well with passenger trains with that speed. So they'd have to have 3 or 4 tracks, 2 fast ones and 1 or 2 slow ones.

Montana does not want to elaborate on the possible rand-value of building such a rail link, which will require quite a bit of tunnelling through the Drakensberg.

Some mountains halfway in between Jo'burg and Durban.

I'll assess the feasibility.

Jo'burg - Durban is about 354 mi / 569 km, rather respectable by HSR standards
Jo'burg - Cape Town directly is about 869 mi / 1398 km, a bit too far. The most populous town in between, Bloemfontein, is not very populous.
Durban - Cape Town is even farther, though the most populous intermediate town, Port Elizabeth, is somewhat better.

Populations:
Jo'burg: 6.3 m / Pretoria: 2.3 m / Gauteng Province: 10.5 m
Durban: 3.5 m
Bloemfontein: 0.37 m
East London: 0.48 m
Port Elizabeth: 1.2 m
Cape Town: 3.5 m

i would agree freight would be necessary to make these viable . but not ordinary freoight trains , high speed lightweight ones . There's no reason why they cant travel at the same speed as the passenger trains , though i would imagine lower speed may redue fuel consumption considerably .
so we you think freight , think federal express / dhl type freight , rather than a coal drag .

Something like SNCF TGV La Poste (Wikipedia).

But for the more usual sorts of freight, keeping axle loads down will require a lot of trains. The TGV's are designed with axle loads less than 17 metric tons per axle, so their tracks will stay in alignment longer and require less maintenance.

I am pretty sure the whole of the existing line to Durban is electrified (check with Thomas Cook, yes, it is). In 1969 I took a train from somewhere near Lesotho to Durban. My impression is that it was electric the whole way. If he intends to build a standard gauge line then quite possibly the freight will continue to go on the old line. But is there enough passenger demand for many trains a day on a High Speed line?
Newly planned lines in Africa are often proposed at standard gauge. (see also Nigeria and Sudan).