You switch at Marlboro. I checked this for myself recently (and realised I would have to drive to the airport to make my flight): earliest you would get to the airport is shortly after 6. The first train leaves Hatfield at 5:26 and arrives at Centurion about 10 minutes later. Then there's a wait of a few minutes at Marlboro. Timetables downloadable at join.gautrain.co.za . Also (nice and clear) on the app, which is otherwise fairly limited.
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