I worry about Chinese intrusion into HK life generally - but THIS seems to be a furfy without real value to the argument. Nobody is getting on that train to travel to somewhere in HK. They are going to CHINA. Once they get on the train, they have accepted that by the time they get off, Chinese law applies. So the wearing of a dissident T-shirt example, surely the guy will have to take it off before they get off the train so it's only a matter of a few minutes ffs.
In this instance, convenience and common sense trumps irrational fear.
* if there is any instance where a HK person can get on the train and get off in HK, for example at that other terminal in NT, it would be an issue......