Diamond Jubilee Tube train was faulty
A Tube train that broke down in a tunnel stranding 733 passengers was a special Diamond Jubilee service, Transport for London (TfL) has said.
Passengers were led down down the track after the train stopped on the Jubilee line at 17:35 BST on Wednesday near St John's Wood, in north-west London.
TfL commissioner Peter Hendy said the system "won't be perfect, but it will be OK" for the Olympics.
The train bears bunting decorations for the Queen's Diamond Jubilee.
Jennifer Siggs, 27, from Acton, west London, was one of the passengers stuck underground for more than four hours.
She saw one woman slumped on the ground who appeared to be struggling to breathe in the heat. She and another woman, a diabetic, were among those given first aid.
'Can't be happening'
Ms Siggs said: "It was so frustrating. For the first hour the driver kept saying we would hopefully move shortly. The driver was talking so quietly and we couldn't hear him over the ventilation units."
She said that the police, paramedics and TfL staff had been very helpful but the situation had been "ridiculous".
She added: "You kept thinking, 'This can't be happening'."
The Met Office recorded the surface level temperature at Hampstead at 24C (75F) when the train broke down.
London Ambulance Service said two ambulance crews, three duty managers and a hazardous area response team had been sent.
They treated a 27-year-old woman, but would not specify what was wrong with her. She was taken to the Royal Free Hospital, while two others were treated at the scene.
A TfL spokesman said the 733 passengers would receive £40, and fares would be refunded for anyone on the Jubilee line during the disruption.
Nigel Holness, London Underground operations director, said it was "clearly not the level of service customers have a right to expect".
"Our staff tried to move the train. This was not successful and we subsequently took the decision to walk passengers off the train," he said.
Mr Hendy said the same "major failure" affected two trains at the same time.
He responded to criticism, saying: "There's no transport system on earth that will perform perfectly all the time. We're working very hard to make it as reliable as possible."
But London Assembly Member and Labour's transport spokesman Val Shawcross said she feared London could end up "an international laughing stock" when the Games are held.
She said: "The Olympic Games opening ceremony is now less than nine weeks away and still ordinary commuters are being hit by delays caused by faulty track, faulty trains and faulty signals.
"Passengers on the Jubilee Line have endured months of shutdowns for maintenance and upgrade works that were supposed to stop this happening.
"So why are there still issues with the line? What will happen if the chaos happens during the Olympic Games?"
Mr Hendy apologised for delays, telling the BBC it was "very unsatisfactory".
But he said the experience was not typical of a line that operates 30 trains an hour and is "becoming more reliable".
"Yes, there's a question about how well the system will perform during the Olympics, and the answer is, it won't be perfect but it will be OK.
"And actually, if you look at last night, every one of the Olympic venues along the line was served during the entire period of the failure."
He said London's transport network was "very dense" and part of its resilience was that "when one piece of it fails the rest of it is sufficient to carry us through".