or allow the gautrain trains to travel on ordinary (prasa) tracks?
I mean at least then the route network might be larger.
That's not possible, gautrain uses standard gauge 1435mm wide tracks while the rest of SA runs on 1067mm cape gauge.
https://en.wikipedia.org/wiki/Standard-gauge_railway
When crossing borders between some countries with different gauges they can do a bogey swap on the carriages but this is obviously not practical on short routes.

