PTC ... yes, I get the sarcasm, but thinking about it you have a point. My local light rail system has signals that do in fact override the driver and stop the train if ignored - what happened here? Are there no signals on this tram system? That would be very surprising for a modern British line. So why didn't they automatically slow or even stop the train well before the turn?

Given the description of the line in the articles, a real possibility might be loss of traction. Apparently it was raining heavily at the time, and the line has a steep downgrade leading to a very slow-speed turn: a great opportunity for loss of control given slippery rail and any lack of driver attention.