Gautrain is the safest railway system in South Africa. So, I think it's worth expanding to serve not only for passenegrs between O.R. Tambo International Airport and Johannesburg or Pretoria but also for passengers in other suburbs of Johannesburg.
On the other hand, Metrorail, which is the conventional commuter rail system in South Africa, is not preferred as a transportation tool, due to its poor safety. Safety here means "Crime on trains".
Unless safety issues on Metrorail trains are solved, there would be no choice but to expand Gautrain networks.
