Formal language is very precise and accurate, avoiding any form of colloquialism. This passage certainly is definitely not formal, and also doesn't contain jargon or slang.

Which means that it's an example of colloquial writing, which uses a chatty style with perhaps the occasional slang expression and maybe a bit of jargon.

Brackets denote PARENTHESIS which is a useful way of adding a little bit of information which is not absolutely necessary for understanding the rest of the passage.

This is a very precise measurement which had precise meaning at the time of its creation. It was the standard width used for most railway lines throughout South Africa. The metric equivalent would be irrelevant in meaning.

The earliest railway line was from Cape Town and through a very mountainous region. A narrower gauge line was useful because of the sharp curves.

Only later were railways built elsewhere in South Africa (e.g. from Natal to the Transvaal border and in the Eastern Cape) but they were deliberately designed so that different routes could be joined together and carriages interchanged.

The same gauge was thereupon continued through most of Africa for that same reason.

The coaches themselves are built just as broad as their overseas counterparts. It's only the gauge of the line itself that is narrower. The trains are therefore slower but just as comfortable.

The Gautrain runs from Johannesburg to Pretoria and includes the Oliver Tambo Airport. The purpose of this line is a high speed commuter train, and high speed necessitates a wider gauge.

The obvious disadvantage is that trains designed for the narrower gauge can't continue on this newest track.

You answer needs to include: the gauge is the same as horse-drawn carriages, which in turn used the same gauge as Roman chariots. Each of these had to have a standard gauge because they made ruts in the road (as one finds ruts made by motorcars in rough roads through the veld). When travelling at high speed, these vehicles would need the parallel ruts to be of a standard width.

Irony is a statement which contains an unexpected opposite meaning. The irony in this passage is that one wouldn't expect a modern high speed railway system to have been designed with the same dimensions as ancient Roman chariots.