A hundred years ago there was a belief, resulting in a policy, that non standard railways were suitable for lines in places with small demand for transport. Mainly there was a belief that construction cost were less than for standard gauge. As a result a number of narrow gauge railways were built. All of them lost whatever economic advantage they might have had when powered road transport developed. A few remain where they have developed passenger services. I am thinking of a narrow gauge branch in Stockholm which has frequent service, and the speed limits are not a drawback. For freight their main disadvantage has been the need to tranship when linked with standard gauge lines. But for long distances, as in Africa, the main disadvantage is the limits to speed. Even in South Africa the Cape Gauge lines were limited, though their developed country engineering had pushed the speed limits up - at least on some lines. There is a very good reason why they are being replaced in many parts of Africa. Australia and New Zealand have similar situations as in South Africa, but in Australia there has been a policy since the 1930s of replacing narrow gauge with standard, as for example on the trans-continental link to Perth.