At least eight people are confirmed dead and 160 injured after two trains collided near Paddington Station in west London at the height of the morning rush hour.
Thirty-nine people are seriously injured, with three critically ill and 11 in intensive care.
There are thought to be some 200 with minor injuries.
Many more passengers could still be trapped in the wreckage, said Chief Inspector Brian Gosden of British Transport Police.
Hospitals across the capital are taking in casualties. St Mary’s Hospital has cancelled all out-patient appointments to treat most of the wounded.
The walking wounded are being treated at a local school and Sainsbury’s supermarket.
A Thames Trains 0806 BST from Paddington to Bedwyn in Wiltshire collided with the incoming 0603 BST Great Western 125 express train from Cheltenham at 0811 BST.
Carriages came off the track and one train burst into flames. A massive column of smoke could be seen across west London.
Up to 30 ambulances, 12 fire engines and 70 firefighters rushed to the scene.
Mark Rogers, a passenger on the 0806, said: “There was an almighty crash and the train rolled over and over, first onto its roof and then onto its side.
“One woman I saw thrown out of the window and she was trapped beneath the train. She was at least very severely injured.
“It is absolute pandemonium. ”
Mr Rogers said the carriage behind the driver had been “ripped apart like a sardine can” and was lying over the top of the 125 Great Western train.
“It is chaos. There are doors and broken glass lying everywhere.”
Prime Minister Tony Blair said: “I am absolutely appalled by what is a truly dreadful tragedy.”
Transport Secretary John Prescott has promised a public inquiry into the crash and praised the “swift efforts of all the emergency services”.
Conservative Transport spokesman John Redwood said: “We need a proper inquiry and we need some answers for the future.”
The trains collided on the same stretch of line as the Southall rail crash in 1997, in which seven people died and 150 were injured.
Health and Safety Executive inspectors are at the scene.
What’s Happened Since The Crash Occurred
Investigations revealed how 31 people died and dozens were injured because of a head-on collision when one of the trains passed a red signal.
Public inquiries were headed by Scottish judge Lord Cullen. He made dozens of safety recommendations and concluded Railtrack, the company then in charge of rail infrastructure and its investment, had failed to respond to earlier warnings about signalling problems.
He also criticised the Health and Safety Executive’s Rail Inspectorate and recommended an independent Rail Industry Safety Body.
Another inquiry comparing the Ladbroke Grove disaster with the 1997 Southall crash recommended implementing sophisticated safety technology.
Thames Trains was fined a record £2m in April 2004 for breaking health and safety laws. The court was told the train driver had not been warned about problems with the signal at Ladbroke Grove and had not received adequate training.
In December 2004 the Paddington Survivors’ Group complained that many of the safety recommendations made after the crash in 1999 had still not been implemented.
A year later, in December 2005, the Crown Prosecution Service said no individuals would face criminal charges over the crash as there was “insufficient evidence” to provide a realistic prospect of conviction”.
In October 2006, Network Rail admitted health and safety breaches concerning the siting of the signal. It received a record fine of £4 million for its part in the crash on 30 March 2007.
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