Behind the scenes: Signalling

Man monitoring Northern line signalling

We’re increasing capacity, improving reliability and making journeys faster 🏃‍♀️ by installing new signalling on our lines. Find out about Tube signalling and why it’s such an important part of our infrastructure.

What signalling means

Signalling keeps 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. Each block has a signal at its entrance.

The average length of each block on the Tube is about 300 metres. In order to prevent a train from entering an occupied block, each signal has a mechanical ‘trainstop’ next to the track. This device applies the brakes of any train which tries to pass a stop signal.

The meaning of a ‘signal failure’

Signals fail for many reasons. 😢 In some cases, the system itself is working normally, but the equipment has detected a problem with the track. Because signals are designed to ‘failsafe’ whenever a fault occurs, signals turn to red 🚥 and trains stop running.

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 open-air sections of Line.

We run a high frequency service and 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.

What we’re doing to improve our signalling

We have an ongoing programme of work to reduce signal failures. It includes replacing old track, improving drainage, cleaning block joints regularly and also improving the insulation between track sections with a new type of rail joint. 👍

We’re also upgrading signalling systems across the network. When London Underground opened in 1863, the signalling was manually controlled at each station by signal boxes. These signal boxes directly controlled the signals and points in the area. New signalling systems are computerised and centralised into control centres 🏢 covering much bigger sections of the network.

Watch our video 🤓 to find out more about how signalling is changing.

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