A major disruption on the UK’s rail network has caused delays on at least nine lines on Friday morning.
National Rail said the issue was due to a “nationwide fault” with the radio system used between train drivers and signallers that has now been fixed.
The Elizabeth line, Gatwick Express, Great Northern, Northern, ScotRail, Southeastern, Southern, South Western Railway and Thameslink services have all been affected.
This includes delays to journeys in and out of some of England’s major transport hubs – such as Manchester Piccadilly, London Paddington and Southampton Central.
National Rail said travellers should plan for some services to be cancelled at short notice.
Travel expert Simon Calder told BBC Radio 5Live commuters should “hope for the best but be prepared for delays and disruption”.
He added that Friday morning had been “a miserable rush hour for a lot of people”.
National Rail said the issue appears to be a fault with the onboard GSMR radio system, which is used to communicate in case of emergencies.
The BBC understands the cause of the fault has been located and has been fixed.
To ease disruption prior to this, a back-up system was being used instead.
Network Rail said the issue was mainly affecting trains at the start or end of the day, which a spokesperson said was “causing a few minutes of delay at the start of the day”.
But “once up-and-running the system is working normally and the impact on passengers is minimal,” it added.
Even though the problem has now been fixed, National Rail said there may be some knock-on disruption.
It advises that passengers may be entitled to compensation if their journey is delayed.
Across the network on Friday morning, most delays appeared to be no longer than 15 minutes, but some services have been delayed by up to an hour.
National Rail warns there are still delays of up to 15 minutes on Southern, Thameslink, Great Northern and Gatwick Express services, with some short-notice cancellations possible.
Most passengers on these lines will be able to travel on their normal route, National Rail said.
Some Heathrow Express trains between Reading and Heathrow Airport have also been delayed, along with Elizabeth line services across the entire line.
ScotRail said its services were affected on Friday morning but have since returned to normal operation.
Earlier, the Gatwick Express was only running between London Victoria and Gatwick Airport, not serving Brighton.
Elsewhere, South Western Railway trains from Brighton to Southampton Central were only running to Bognor Regis or Chichester, while Great Northern services between Kings Lynn and London Kings Cross were only running between Kings Lynn and Cambridge.
The issue was caused by the railway’s GSMR system, which allows drivers and signallers to communicate in areas – such as tunnels – where previous analogue systems did not work.
It works in a similar way to mobile phones, but the digital system is not reliant on commercial mobile phone operators. The rail network has its own contained phone and mast system.
One rail source told the BBC that “the system [wasn’t] connecting easily”.
The BBC understands that a new piece of hardware, installed as part of an upgrade to the system at a major telecommunications hub in Stoke, was the source of the problem.
Replacement hardware has been installed and the system rebooted.
While that was being fixed, we were been told that there were no safety-critical issues, as staff were using a workaround to manually reconnect.
This well-rehearsed backup is where drivers enter a code provided to them on a “wild card”, which lets them establish communications with the national network.
It has been described to us as working like a wi-fi password – when the correct code is entered, the system becomes fully operational and will remain so.
Radios that had been manually programmed by drivers are now being switched back to automatic mode.
