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.

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.

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 proceed. The Central line is mostly run like this.

Communications-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 utilising high-resolution train location determination, independent of track circuits." In other words, the system knows where the trains are at any one moment and can therefore optimise performance, allowing as many trains as possible to operate.

Unfortunately, as commuters have discovered, it is not infallible. Sometimes trains go "non-com" for no apparent reason - as happened on the Northern line. Anything from systems failure to a cyber attack can bring trains to a halt.

Richard Jones of TfL said on Friday: "We have completed several interventions to get the system operational but we do not yet have full certainty on what caused the failure, so further investigation work is needed before we are sure of a full resolution."

There is an analogue default: drivers being "talked through" by controllers to ensure trains are a safe distance apart. But that drastically cuts the number of trains that can run.

A major programme is underway to upgrade the 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 are just below street level. TfL says that two years before completion, passengers are already benefiting from faster journey times.

Although the east-west line previous known as Crossrail shares stations with the London Underground, it is not part of the Tube network. The central section uses CBTC, but on Network Rail stretches that means trackside signals. The aim is that the European Train Control System (ETCS) will eventually be used throughout Britain's network, allowing more trains to run safely and accelerating journeys.

Where to start? As the world's oldest subterranean rail system, the infrastructure is always in need of improvement. On the Piccadilly line, many closures are planned to allow for upgrading the line. For example on 1 and 2 November the stretch serving Heathrow airport is closed.

On the Northern line, meanwhile, the new year will see partial closure: "Trains will not run between Camden Town and Kennington, via Bank, after 10pm Mondays to Thursdays from Monday 12 January until late spring 2026.

This subject has been much discussed, but the prevailing view is that an operator should always be at the front of a train in case of unforeseen events.

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