Signalling is needed to keep trains a safe distance apart. Railway lines are divided into sections called blocks and only one train is allowed into one block at a time, protected by a signal at its entrance.
Low voltage current is supplied to the running rails which, when short-circuited by the passage of a train's wheels, operate relays that control signals.
The average length of each block on the Underground is about 300 metres. To prevent a train from entering an occupied block, each signal has a mechanical 'trainstop' next to the track. This device will apply the brakes of any train which tries to run past a stop signal.
Signals fail for many reasons. In some cases, the signalling system itself is working normally, but the equipment has detected a problem with the track. Because signals are designed to 'fail safe' whenever a fault occurs, signals turn to red and trains stop running.
As the Tube's signalling system uses small electrical currents in the track to detect the movement of passing trains, signal failures sometimes happen when there is a short circuit between the running rails. These short-circuits may occur after heavy rainfall, when puddles of water build up on the track - particularly on our open-air sections of line.
Also, with our high frequency of service, the accumulation of iron filings (from the daily wear and tear of trains) across insulated joints between sections of track may also cause problems. There have even been cases of rodents chewing through cables, turning signals to red.
In order to reduce signal failures we are replacing old track, improving drainage, cleaning block joints at regular intervals and improving the insulation between track sections using a new type of rail joint.
We are also upgrading signalling systems across the network. When London underground opened in 1863, the signalling was controlled at each station by signal boxes. These signal boxes directly controlled the signals and points in the area. New signalling systems computerise and centralise these boxes into control centres.
On the Jubilee line, new signalling has allowed us to run more frequent trains - now 30 every hour - carrying 12,500 extra passengers an hour on a service that's 50% more reliable.
The Northern line signalling system has now been modernised, delivering up to 20% more capacity or space for an additional 11,000 customers per hour.
And we have plans to push frequencies on the Jubilee, Victoria and Northern lines even further.
A new signalling system on the sub-surface network - the Circle, District, Hammersmith & City and Metropolitan lines, which together make up 40% of the Tube network - will increase capacity by a third.
Find out what Tube signals actually are and how we are upgrading them to provide you a better service. Find out why this sometimes takes longer to complete. Browse through our videos on YouTube to find out more. #TubeImprovements
Watch our video on YouTube to find out how the signalling modernisation on the Northern line benefits you.