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Passengers are being advised to avoid one of the busiest lines on the London Underground after its signalling system was hit by a mystery fault. The Northern line has been blighted with signal failures near Stockwell station since Sunday evening that have meant far fewer trains than normal have been able to run on the line. This has caused delays and knock-on congestion on trains, even at off-peak times. On Thursday lunchtime, The Standard witnessed passengers battling for space on overcrowded train at Bank and Moorgate stations. Despite calling in specialists, Transport for London admitted there was no end in sight as it had been unable to identify what was causing the problem. A TfL spokesman said on Thursday that the cause of the problem remained unknown, despite engineers having been on site since Sunday evening. Stockwell is normally served by about 15 Northern line trains an hour in each direction but this has fallen to between six and eight trains an hour. This means that rather than waiting up to our minutes for a train, the gap can be as much as 10 minutes between services. At some stages there have been only four trains an hour, meaning a 15-minute service. The problem means that line controllers are having to use two-way radios to guide Tube drivers through the tracks around Stockwell because they cannot rely on the signalling system advising whether the track ahead is clear and that it is safe to proceed. This means the trains have to travel at slow speeds – creating a bottleneck effect and limiting the number of trains across the entire line, in both directions. The ATL signalling system has been in use on the Northern line since 2012. It is also used on the Jubilee line. This is the first time this type of fault has emerged. When The Standard travelled to Stockwell on Thursday, there was a 14-minute wait for a southbound train to Morden at Old Street station. The train was then held for several minutes at Kennington to “regulate the service” – adding to the journey time. A number of northbound trains from Stockwell terminated at Archway rather than High Barnet, probably to keep as many trains operating in the central section. Northbound trains were arriving with gaps in the service of between five and 16 minutes. TfL’s online journey planner continued to warn of “severe delays” across the line and advised passengers: “Please avoid the Northern line where possible. You will find it quicker to use alternate routes if you can.” In an email to passengers, TfL warned that the problem could potentially continue “until later in the week, due to the complexity of the failure”. The problems were said not to have been linked to a global internet outage on Monday. Nick Dent, TfL’s director of customer operations, said: “We apologise to customers affected by delays on the Northern line since Sunday. “We have identified a fault with the signalling equipment around the Stockwell area that means our service controllers need to manually talk trains through this part of the network. “Our engineers are working hard to identify and fix the fault, including deploying advanced test equipment, and we will do all we can to restore a good service as soon as possible. “We are advising customers to allow extra time for their journeys, and to check before they travel using TfL Journey Planner or the TfL Go app.”
