Lineside signals are fast disappearing across the London Underground network, being replaced instead by various forms of Automatic Train Operation (ATO), with only the Bakerloo, Piccadilly and bits of the Metropolitain Line still largely untouched by automation.
Signals played a critical role in the safe running of the main railways and the first signals displayed painted wooden boards on posts. Early signalling on London Underground also used these 'semaphore' signals but LU was an early adopter of colour light signalling. The cabin at Epping went live with colour lights when the track was electrified in 1949, replacing an old mechanical signal box on platform 2 built by the Great Eastern Railway in the days of steam services.
Most colour lights were controlled from the signal cabin with the familiar cables running alongside the railway. Some colour lights could however change automatically with the passing of a train. Points were nearly all controlled electrically, not by using motors, but by using compressed air. An electrically controlled air valve released pressured air into a piston which in turn operated a crank to move the point blades into position.
We have a range of signals at the Museum and our latest exhibit is a working set of 'arbour' lights - the three white lights above the main signal LW3. When lit, these showed the train driver that his train was being signalled onto a diverging route. Other signals on display include disk signals, calling on signals, theatre lights, coffee pot signals, repeaters and, although not a signal in the true sense of the word, a Rail Gap Indicator. These RGIs would only light up red if the power to the rails in the section ahead was turned off for some reason. It would be a foolhardy passenger train driver who went past a RGI because his train would surely stop when it ran out of electricity supply!
Epping Museum was fortunate to be donated a set of points (catch points to be precise) when the track towards Ongar was modified and power rails removed. Powered by an air compressor under the signal cabin, we can demonstrate how the air operated point blades can be effortlessly 'blown' to a different position. To anyone who lives near a terminus or depot, the hissing and clunking of points changing will be a familiar sound.
Unlike their counterparts on main line railways, discs signals too are also controlled by compressed air, the circular disc being rotated by the action of an air piston. You could be forgiven for thinking that LU signallers had a whole lot easier job than their colleagues in manual signal boxes!