When Elon Musk first proposed a transport system that would whizz pods full of people from city to city through elevated tubes at the speed of an airplane, it sounded gloriously eccentric.

It would be enormous fun, of course, climbing on board in Johannesburg and popping up in Cape Town a few minutes later. But there was so much technology to perfect, huge expense in building the infrastructure, and the inconvenience of needing massive pylons to support the tunnel as it snaked across the country.