OK, let's try a little experiment.
Suppose you're in charge of the London Underground, and you're having the slight annoyance of an occasional passenger being run over by a train.
You could:
1. play a recording saying "Mind the gap" every ten seconds or
2. put a wall with sliding doors between the platform and the train.
Which one would you choose?
Both
. same as they do in HK,
because there is always some Deaf guy who cannot hear the announcements
then they have people standing by the walls to stop the mainlanders trying to force the doors open, and Security for the people like (gox) who flatly refuse to follow the rules