Major disruption is expected on some rail services in southern England until the end of the day after an earlier radio fault, National Rail has said.
South Western Railway, CrossCountry, Southern, Gatwick Express, the London Overground, Great Western Railway and Thameslink all saw cancellations and delays on Thursday morning after the issue affected communications between trains and signallers.
National Rail said the problem had been reported at 08:53 BST and resolved by 11:00, with services starting to return to normal.
But it warned: "Some services may still be delayed by up to 90 minutes or cancelled whilst service recovers. Major disruption is expected until the end of the day."
Shortly before 17:00, it said Southern, Thameslink and Gatwick Express were no longer affected. The situation had already been resolved on the London Overground by midday.
Those travelling on South Western Railway, CrossCountry and Great Western Railway at rush hour and into the evening were warned disruption was still possible.
Passengers were urged to check before travelling and advised of alternative routes on which their tickets would be valid.
South Western Railway (SWR) - which had warned of delays of more than an hour, cancellations and alterations across its network earlier in the day - said "significant" disruption would continue as it worked to get services back to normal.
It warned trains could be held in stations as well as being delayed or cancelled, and timetables and platform information could be changed at short notice.
Passengers could use their tickets on Friday, it added, or at no extra cost with other rail companies, buses or the London Underground on Thursday.
Transport for London urged commuters to avoid London Waterloo station - a major rail hub - with long queues and crowds pictured at rush hour.
It warned the station would be "extremely busy" due to the disruption and a separate track fault on the Jubilee line.
SWR also advised its services were affected by an "unrelated signalling problem" at nearby Wandsworth Town.
Southern Railway had said early on Thursday that its services to and from London Victoria were particularly affected, as well as its West Coastway route between Brighton and Portsmouth/Southampton. It warned journeys could be delayed by an hour into the afternoon.
Gatwick Express had likewise warned of delays for services running to and from London Victoria, while Thameslink said there would be likely delays on its services running to and from Brighton, Horsham, and Three Bridges.
GTR - which operates all three companies - apologised to customers for the disruption, which it said had affected its services west of Chichester, West Sussex.
Great Western Railway said just after midday that its services were "no longer affected" and had returned to normal, although National Rail advised disruption was still possible.
One of those affected was Gemma Givans, 28, whose train from London Victoria for a work trip was cancelled.
"If I don't work, I don't get paid," the tattooist told the BBC, adding that trains had been "really inconvenient" as of late. "It is affecting me financially."
Sixth-form student Caleb Anderson meanwhile said the delays from Portsmouth meant he would be several hours late for college in Winchester, with a friend coming to collect him in the car.
The 18-year-old said that, with his A-Levels starting in a few weeks' time, it was "stressful".
On a train to Portsmouth, Paul Barrick said he was "not a very happy traveller", as the delays meant he would miss his onward ferry to the Isle of Wight for a 30-mile coastline hike.
"I'm not going to make it," said the 50-year-old from Basingstoke. "I will have to get to the terminal and just hope I can get on a later one."
He said the morning had seen "a lot of inconvenience and, of course, disgruntled passengers".
Guidance is available from National Rail on whether people are entitled to compensation from delayed or cancelled journeys.
Additional reporting by James Kelly, Joshua Askew and Briony Leyland
