Although they can be supplemented by other technologies to counter the less scrupulous traveller, ticket transactions provide an ideal indication of when and where the vast majority of individuals enter or exit the rail network.

Ticket transactions therefore contribute basic intelligence that can support more complex analysis tasks. The tracking software pinpoints where and when suspects have been throughout their journeys, from entering the network to arriving at the crime location to exiting the network.

It can also determine if different suspects could have feasibly been in contact with each other during their journey and where and when that could have taken place.

Haddy points out using facial recognition technology alone to filter through that amount of passengers would not be practical, especially when aiming to capture information on suspects quickly.

"The problem is when cameras have low light or are positioned in lowlight conditions and it's in a crowded environment it's pretty impossible. Even if facial recognition is possible in those sorts of conditions, the amount of computing power required to track millions of passengers would be prohibitive.