Thousands of passengers are experiencing travel chaos after a major signalling fault on the South Western main line.
The operator's services were already being disrupted by the second day of a five-day walkout by members of the Rail, Maritime and Transport (RMT) union.
A fault between Woking and Surbiton means many trains are unable to run through to London Waterloo.
Some services have been cancelled, delayed or revised, with disruption expected until 4pm.
Services between Salisbury and Waterloo will start and terminate at Basingstoke.
Trains are not calling at Walton-on-Thames, Esher or Hersham.
South Western has told passengers their ticket will be accepted at no extra cost on services operated by Great Western Railway, Transport for London buses, London Underground, Southern and Thameslink Railway, the TFL Tramlink, and CrossCountry trains between Bournemouth and Reading.
Tens of thousands of commuters use the 143-mile South Western main line that runs through parts of London as well as the counties of Surrey, Hampshire and Dorset.
Charlotte Neal, who commutes from Woking to Tottenham Court Road, told Sky News: "It was just a complete nightmare, people were packed on the train like sardines and it kept stopping to wait for the driver to get permission to continue.
"It took 40-45 minutes to get from Woking to Clapham junction – usually a 19 minute or so journey.
"Then when everyone got off the train, the platform was so packed at Waterloo people waiting couldn't get on the train.
"Most services were cancelled and the track failure is the last thing people need at the start of a five-day strike.
"It's incredible that despite paying as much as we do, commuters are still faced with this daily drama just to get to work.
"South Western trains is saying not to use its service, but for people working in London it is their only choice."
@SW_Help I gave up on getting to work after waiting at Woking for ages. Trying to get back to BSK to go home (at suggestion of your announcer). Now youve driven an empty train off the platform that was supposed to go to BSK because theres no crew. Weve been told /2
— Jimbo the Zimbo (@zimbo_jimbo) October 24, 2018
Millie Waller, who commutes from Chiswick to Tottenham Court Road, saw a woman faint on one of the packed trains during the delays.
She told Sky News: "With the South Western line from Waterloo to Hounslow being one of the only routes in and out of London for those living in the far west of London, any disruption to the train times has a huge knock-on effect.
"Not only has the strike meant that instead of four trains an hour, we are down to two, but this morning one of those two trains was also cancelled which led to a huge amount of passengers waiting hopelessly on the platform for a train.
"One girl actually collapsed and fainted, and then had to be carried off the train at the next station by two helpful passengers which caused even more delays."
Ms Waller, who also been affected the five-day strike action, added: "Train drivers run a service which we all rely on, I dont know of any other roles that strike like this when they want more money.
"Most of us would get fired for not turning up to work for four days."
Twitter users have expressed their dismay at the delays on social media.
Johnny Hayes wrote: "Come guys.
"I can't even get out of Woking Station this morning.
"Does anyone know what's going on?"
*UPDATE 24/10* Due to track circuit failures, following lines are affected:
> Weymouth/ Bournemouth/ Alton/ Basingstoke/ West of England/ Woking to Waterloo
> Portsmouth to Waterloo via Guildford/Basingstoke
> Dorking to Waterloo
> Guildford to Waterloo via Epsom— SWR Help (@SW_Help) October 24, 2018
South Western said in a message to passengers: "Due to a major fault with Network Rail signalling systems between Woking and Surbiton, trains are unable to run on this line through to London Waterloo.
"We are advising all passengers who travel on the mainline (services from Weymouth, West of England, Portsmouth) and suburban (from Guildford to Waterloo) services that run through these stations, not to travel and to check for updates."
Network Rail has said its engineers are on site and are working to resolve the issue as quickly as possible.
If you are delayed by 15 mins or more at your destination, you are entitled to compensation. Please see here more details and to file your claim online https://t.co/u9dx5QOZDs
— SWR Help (@SW_Help) October 24, 2018
The firm apologised for the disruption and thanked passengers for their patience.
South Western has sent another message to passengers saying all lines in the Woking area and part of Basingstoke station will be closed all day on Sunday for track maintenance.
Members of the RMT began five-day industrial action on Tuesday in a long-running dispute over the role of guards on trains.
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RMT general secretary Mick Cash said: "On the second of the five days of strikes in this current phase of action on South Western Railway, RMT members remain rock-solid and determined this morning in the fight to put public safety before private profit.
"There is only one way to break the deadlock in this dispute and that is through South Western Railway agreeing to RMT's call for talks and sitting down with us to negotiate around the guard guarantee which matches the widespread best practice in the industry."
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