Wi-Fi connection data collection began on 8th July 2019.
Wi-Fi connection data can provide TfL with a far better understanding of how customers move through stations. It is not used to identify specific individuals or monitor browsing activity.
TfL has partnered with Virgin Media to bring free Wi-Fi access to 97% of London Underground stations, giving customers internet access. Find out more about station Wi-Fi.
They are putting up signs at every station explaining the Wi-Fi data collection that is taking place and how to opt-out.
In summary, they are collecting the location of devices on their network, identifying them by a pseudonymised version of their MAC address.
They are trying to understand patterns of movement across their network and how customers as a whole use it, not how specific individuals use it.
They are not collecting data on browsing activity, cookies, phone numbers or whether the Wi-Fi service is used.
However, if you would like to opt-out, you can do this by turning off Wi-Fi on your device, turning your device off or putting your device into airplane mode while at the stations.
Further details – https://tfl.gov.uk/corporate/privacy-and-cookies/wi-fi-data-collection