Transport for London has installed Wi-Fi access points which smartphone devices automatically will request to join. This mechanism provides TfL with data on the location of the smartphone devices. The data does not identify individuals. 

Transport for London are going to use the data in the following ways:

• Wi-Fi data can help to understand the paths people take in stations, the platforms and lines they use, the routes they take when they have many options and the interchanges they make
• Aggregated data can show which sections of the network are crowded, at what times and how this changes in response to events and network alterations
• Data can be used with analytical tools and services to improve the way we run and plan our network, and provide customers with more detailed information

For the following benefits:

• We will be able to give customers better information to help them plan their journeys and avoid congestion. For example, we plan to use aggregated Wi-Fi connection data to show the relative 'busy-ness' of London Underground stations, in near real time.  We will make this available on our website
• We will be able to manage disruptions and events more effectively, deploy staff to best meet customer needs and ensure a safe environment
• We will be able to make better transport planning decisions - for example about timetables, station designs and major station upgrades
• By understanding how many customers we have and how they move around stations (eg the total number of unique devices passing through a zone of a station) we will be able to maximise revenue from the companies which advertise on our poster sites and those who rent retail units on our property. This money can be invested in improving our services

Source: https://tfl.gov.uk/corporate/privacy-and-cookies/wi-fi-data-collection