A full service could soon return to 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 for days with signal failures near Stockwell station since Sunday 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.
But Transport for London said on Thursday afternoon that a better service was “ramping up” - though it remains unsure what has been causing the problem, despite engineers having been on site since Sunday evening.
Tests will continue until engineers establish the root cause of the problems, to learn lessons for the future and guard against the problem appearing on another line.
Stockwell is normally served by about 15 Northern line trains an hour in each direction but this fell to between six and eight trains an hour.
Rather than waiting up to four minutes for a train, some passengers had to wait 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 meant that line controllers had to use two-way radios to guide Tube drivers through the tracks around Stockwell because they could not rely on the signalling system to know whether the track ahead was clear and that it was safe to proceed.
This meant that the trains had to travel at slow speeds – creating a bottleneck effect and limiting the number of trains across the entire line, in both directions.
However, trains were able to operate without radio contact from Thursday morning under a test timetable.
By Thursday afternoon, TfL said it was ready to ramp up the service but admitted this would take time to implement.
Richard Jones, TfL’s director of asset performance delivery, apologised to passengers for the disruption.
“This started on Sunday morning and got progressively worse,” he told The Standard. “Trains on the Northern line were losing contact with the static part of the signalling control.”
The “fail safe” system meant passengers were not at risk but it caused the emergency brakes to activate, brining trains to a stop.
“We have been working 24/7 to try to get to the bottom of this,” Mr Jones said. “We have got specialist signalling and contract engineers. We have had them working overnight and during the day.”
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.
Engineers detected “interference” in the system. Mr Jones said: “On Thursday morning, when we ran the first test trains through, to everybody’s great relief, it performed normally.
“What we now need to do is fully understand which of the interventions made the difference. We still have an awful lot of testing to do, to bottom out what caused this in the first place.”
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.
Until Thursday lunchtime, TfL’s online journey planner warned of “severe delays” across the line and advised passengers: “Please avoid the Northern line where possible.”
But by the Thursday evening rush hour, only minor delays were reported.
TfL has ruled out sabotage, saying the affected equipment was either in a secure location or in deep tunnels.
Nor were the problems 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.”
