Every phone has a unique MAC address, which it sends to Wi-Fi access points in order to connect. TfL will start monitoring the locations of these MAC addresses, which will be tokenized so that they can’t be traced back to the phone, while on the Tube to get a better understanding of crowd levels and how to improve the system.
TfL will not be able to collect data from the phones like browsing history. Users can opt out by simply turning off the Wi-Fi on their devices while on the Tube.