Can authorities track a suspect individual across their journey among a myriad of other people, processing complex data in real time so they know where someone is, or at least has been?

Picture this: The London Underground (the 'Tube') carries 4 million passengers a day. These passengers can arrive at any one of the network's 270 stations, change to any of the 11 lines and other stations at will, and leave by another station (or even the one they entered the network by) with complete autonomy and impunity.

Monitoring this movement of millions are 12,000 surveillance cameras, each potentially sending data every second of every day. Manually tracking just one individual under these conditions -- allowing for varying lighting, crowded platforms and all based around a few pixels on a video image -- is a lengthy and frustrating experience.

Tracking several individuals through different journeys just exponentially compounds the issues. Where to start, where to continue looking, and all that assumes you know who you are looking for.