[rosswilson]

London Underground use a number of signalling systems, here's a good resource to get started learning about them: http://www.trainweb.org/tubeprune/signalling.htm However, ultimately you could develop a super intelligent system using networked PLCs, but at the end of the day you're still going to need relays to actually switch the power to the coloured light signals. The red wires in the photos will lead to a controller somewhere that triggers the relays to switch the mains power on/off to the signals.