Your sentence makes no sense. Clearly transnet is NOT for the public good then. If it had been - it would be reducing long distance road transport, not increasing it.

It's clear that transnet's monopoly is not for the public good. It does not follow that it's impossible for a state-provided service to EVER be for the public good.

The world is full of counter-examples. The SABC is one of the most atrocious quality and badly run broadcasting services in the world. But you can't conclude that any tax/license-funded broadcaster must be similar while the BBC remains one of the best, if not the very best, television broadcasters in the world. 
And when Brits watch BBC they never have to be annoyed by a single ad, they don't have programming choices dictated by ad revenue - which means more niche content can actually be made. 
Whether your speed is Doctor Who or Top Gear or sitcoms that are actually funny - nobody does it better than the BBC. 

Clearly it CAN be done well.