Highly unlikely that it was a survey as they were young chaps and were only on the bridge for 2 minutes max. Maybe they only needed 2 minutes for the survey, but they seemed more interested in taking pics of the view.
I have stayed in the GW2 unit at Skukuza plenty times and one can walk up onto a section of train line from that unit. There’s an old gate with a rusted lock which stops one for walking onto the bridge, but I could have fitted through the gap in I wanted to the last time we were there. Hopefully it’s more restricted now.
I’m hoping that this bridge tender is more about restoration
Anyway, I do know there was some sort of 'official' explanation for the query someone posted on SP forum. I will have another look and see if I can find it.