It will be tapped by GCHQ
Theresa May's snoopers' charter is now well on the way to being law by Xmas. It TfL is providing Wifi then it will be deemed an ISP and thus the data collected must be kept for a year. Web site addresses at first, but expect 'feature creep' in this law (or just plain - share it with us: you have a nice tube network there ...) and they will get the lot.