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

According to operator Transport for London (TfL), the Northern line of the Underground is finally running reasonably smoothly again after four days of disruption.

Richard Jones, TfL’s director of Asset Performance Delivery, said: “I apologise for the disruption and thank customers for their patience while our teams have worked around the clock to resolve the problem.”

But how does signalling work on the Tube, and how susceptible is it to failure? These are the key questions and answers for commuters and visitors to the capital.

How does signalling work on the London Underground?

Signalling is as old as the railways. The aim is to keep trains a safe distance apart and prevent collisions at points where tracks converge.

Historically “fixed-block signalling” has 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 where the block begins.

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, blocks are typically 300 metres long. Circuits in the track detect the movement of trains. In the extremely rare case that the driver passes a danger (red) signal, a device in the track known as the “trainstop” does what you would expect it to, and automatically applies the brakes. Such an event is known as a “Spad” – signal passed at danger.

Sounds sensible. Any drawbacks?

Yes. This analogue system constrains the number of trains that can run, which means the system does not run at its potential capacity. Bluntly, it requires too much “headway” (time between trains). In addition these wire-based systems are susceptible to anything from flooding (when the track circuit can malfunction) to rats or humans damaging or stealing cables respectively.

While fixed-block signalling is still the norm on the Piccadilly, Bakerloo and Waterloo & City lines, other routes have more modern systems.

The next step up, as used on the Victoria line, is Automatic Train Operation (ATO). When this began, in 1968, the concept was revolutionary – with the driver in control of the doors, but a computer deciding how the trains should…