Passengers heading into London faced widespread chaos on the rail network on Friday morning.
But train companies hope the problems will ease before midday and that services will run as normal during the evening rush hour.
From the start of services on Friday, National Rail warned that services on what was the UK’s biggest franchise – the GoVia Thameslink network – could be cancelled, delayed by up to 15 minutes or terminate early.
This affects Thameslink, Southern, Great Northern and Gatwick Express trains.
Southeastern and South Western Railway – which operate commuter trains from south east London and south west London into central London – were also affected.
The Elizabeth line was reported to have problems on its western branches to Heathrow and Reading. Then at 9.30am National Rail reported: “Elizabeth line services across the whole network are impacted by this incident.”
But Transport for London, which is responsible for the Elizabeth line, told The Standard that there was a “good service” across the country’s busiest train line, notwithstanding a track fault at Forest Gate.
This is what we know about what caused the latest problems – and on how long it will take to resolve.
What has gone wrong?
According to Network Rail, trains are delayed because of a “nationwide fault with onboard ‘GSMR’ radio system that allows communication between train drivers and signal operators”.
The GSMR, or global system for mobile communications – railway, is a relatively new piece of safety kit, and cost £1.8bn to install.
The technology uses digital communications between trains and signallers to improve safety for passengers.
Its introduction enabled analogue radios to be discontinued. The system enables train drivers to communicate directly with signallers, including in tunnels and deep cuttings, where the old radios would previously cut out.
According to the BBC, the fault lies within the system at the national telecommunications hub in Doncaster. It failed to connect on start-up in the early hours of the morning.
Using a customised fixed telecoms network alongside mobile technology, its processes include journey registration, operational messaging and prioritising driver-signaller communications. Those involved range from drivers and signallers to managers, controllers and maintenance staff.
What are the implications for passengers?
The issue mainly affected trains leaving the depot to start their service. Once they are in position on the so-called network “diagram”, trains can run normally.
As such, the network gradually began to recover throughout Friday morning.
A Southeastern spokesman said: “All lines are open, but some trains are running between five and 20 minutes late. We expect most services to return to normal by 11:30am.”
Thameslink services between Luton and Brighton were running on time by 10am. There were delays of up to 15 minutes at Blackfriars.
It said problems would continue “until further notice” - but sources suggested the problem was not as bad as feared.
By mid-morning, only one Great Northern train from Moorgate to Stevenage had to be cancelled.
Southern trains out of Victoria were all scheduled to depart on time.
Can passengers get compensation?
Yes – depending on how long they were delayed. The “delay repay” system typically kicks in after delays of 15 minutes – with passengers able to claim back 25 per cent of the cost of their (single) ticket.
On networks such as Southeastern and Thameslink, refunds worth half of the ticket are available for delays of between 30 and 59 minutes. A full refund is offered for journeys delayed by an hour or more.
Does this mean more chaos on the Elizabeth line?
TfL says not. Despite warnings on the National Rail website, TfL insisted there was a good service across the line.
Last week the there was a two-day meltdown on the central section of the Elizabeth line, which TfL commissioner Andy Lord publicly blamed on the line’s software provider Siemens.
