London Underground signalling: how does it work and what can go wrong?

According to operator Transport for London (TfL), northern line of underground After four days of disruption it is finally running quite smoothly again.

Richard Jones, TfL’s director of asset performance delivery, said: “I apologize for the disruption and thank customers for their patience while our teams worked around the clock to resolve the issue.”

But how does signaling work on the tube, and how sensitive is it to failure? These are the main questions and answers for travelers and visitors to the capital.

How does signaling work on the London Underground?

Signaling is as old as railways. Its purpose is to keep trains at a safe distance and prevent collisions at the points where the tracks meet.

“Fixed-block signaling” has historically been the default to ensure safe braking distances are maintained. The line is divided into blocks, and only one train is allowed in a block at a time. Each block is protected by a signal located at the location where the block starts.

In the early days of the London Underground, these were manually operated “semaphore” signals as still seen on heritage railways (and even some Network Rail lines). But now lights are used.

On the underground, the blocks are typically 300 meters long. Circuits installed in the track detect the movement of trains. In the extremely rare case that a driver crosses a danger (red) signal, a device in the track known as a “trainstop” does exactly what you’d expect, and automatically applies the brakes. Such an event is known as a “spade” – the signal passed at danger.

Seems sensible. Any inadequacy?

Yes. This analog system limits the number of trains that can run, meaning that the system does not run at its potential capacity. Obviously, this requires much more “headway” (time between trains). Additionally these wire-based systems are susceptible to anything from flooding (when track circuits can become damaged) to damage or theft of cables by rats or humans respectively.

While fixed-block signaling is still the norm on the Piccadilly, Bakerloo and Waterloo and City lines, there are more modern systems on other routes.

The next step, as used on the Victoria Line, is Automatic Train Operation (ATO). When it debuted in 1968, the concept was revolutionary – the driver controlled the doors, but it was the computer that decided how the train should move. The Central Line is mostly run in this manner.

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Something more 21st century?

Communication-Based Train Control (CBTC), which runs on the Jubilee and Northern lines. The international standard IEEE1474 defines it as “a continuous, automatic train control system using high-resolution train location determination, independent of the track circuit”. In other words, the system knows where the trains are at any given time and can therefore optimize performance, allowing as many trains to operate as possible.

Unfortunately, as travelers have found, it’s not infallible. Sometimes trains become “non-com” for no apparent reason – as happened on the Northern line. Anything from a system failure to a cyber attack can stop trains.

TfL’s Richard Jones said on Friday: “We have completed a number of interventions to get the system back up and running but we do not yet have full certainty as to what caused the failure, so further investigation work is needed before we can be confident of a full solution.”

There is an analog default: controllers “talk” to drivers to make sure trains stay at a safe distance from each other. But this leads to a huge reduction in the number of trains running.

A major program is underway to upgrade four “sub-surface” lines. This quartet – Circle, District, Hammersmith & City and Metropolitan – comprise the oldest lines on the Underground, and unlike the deep Tube lines, lie just below street level. TfL says that with two years to go before completion, passengers are already enjoying the benefits of faster journey times.

What about the Elizabeth Line?

Although the East-West line formerly known as Crossrail shares a station with the London Underground, it is not part of the Tube network. The central section uses CBTC, but on the Network Rail section this means trackside signals. The aim is that eventually the European Train Control System (ETCS) will be used across the entire UK network, allowing more trains to run safely and making journeys faster.

Any other issues faced by London Underground users?

Where to start? As the oldest underground rail system in the world, the infrastructure is always in need of improvement. On the Piccadilly line, a number of closures are planned to allow the line to be upgraded. For example, on 1 and 2 November the portion serving Heathrow Airport is closed.

Meanwhile, on the Northern line, partial closures will be seen in the new year: “Trains will not run between Camden Town and Kennington via Bank after 10pm Monday to Thursday from Monday 12 January until late spring 2026.

Will we ever see driverless trains on the London Underground?

There has been much discussion on this topic, but the prevailing view is that an operator should always be at the head of the train in case of unexpected events.

Read more: Not just double-decker trains: how the Channel Tunnel could change rail forever