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

Deep trouble: London's Underground is the oldest in the world - (iStockPhotos/Getty)

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.”