According to shanskraal,
[Thomas Burgers, president of the Transvaal Republic] considered a railway link with the non-British port of Delagoa Bay (Maputo) as essential for the economic survival of the country.
This was in the mid-1870s when the Transvaal was on good terms with Britain and the Cape Colony. Later, in the early 1890s when relations had greatly deteriorated, the Transvaal under the hardline Paul Kruger explicitly sought a railway link to a port not controlled by the British, for strategic reasons. But I am surprised they would be thinking along those lines already in the mid-1870s under the moderate and progressive Thomas Burgers.
Was the Transvaal in fact already thinking about the strategic advantages of a link to the sea that didn't pass through British territory?
Or were there geographical reasons for preferring to go east instead of taking the apparently more economically useful route south through the Orange Free State and the Cape Colony?