Well, my journey to work yesterday was ruined, like those of many others, by a signal failure at High Street Kensington, so I thought I would do a bit of research into how the Tube's signals actually work. Here are the results.
Pictured is G-10x signal unit, typical across the network:
Its components are:
A: The master signal control. The linchpin of the unit, the master control establishes what trains are on the line and which way the points are set. In a design that combines durability with resourcefulness, most of them are made from parts scavenged from toasters.
B: Signal relay board. This is the mechanical heart of the unit and provides back-up in the event of electronic failure. It's made from high-grade British plasticine.
C: Electronic control box (inset picture). Similar to those used in the reactor cores of the old Magnox nuclear power stations, this tangle of wires might not look like much but it was cutting-edge electronics in 1971, capable of providing computing power equivalent to a modern wristwatch. Once the Tube upgrade is completed in 2008, it will provide computing power equivalent to a modern digital watch. The wires are mainly surplus from the Romanian tractor industry.
D: I'm not entirely sure of the usefulness of this part. It's a yoghurt pot painted silver and stuck to the side of the unit with Pritt-Stick. It must do something important, because it's on most units.
E: System anchor. This ensures the unit isn't shaken to bits by vibrations caused by passing trains. It's held together by top-quality chewing gum (inset picture) and guaranteed by Wrigley's. Only Spearmint will do, unlike lesser networks that use Juicy Fruit.
F: Signal switch hub. This passes information from the signals back to trains and control rooms. It's a triumph of British design and miniaturisation: the box may look small, but the Tube engineers source especially small people to sit in them, pulling levers.
So don;t curse the next time you hear about a signal failure. These high-quality precision machines work faultlessly for hours on end, and failure rates have fallen significantly since the last balsa-wood components were eliminated.