Southall, 1.15pm, Friday 19 September 1997. It's happened again.

Six people died, 13 were seriously injured and a further 150 were described as "walking wounded" yesterday after one of Britain's worst rail crashes. Steve Boggan, Kathy Marks, Randeep Ramesh and Christian Wolmar describe the tragic scene and examine the reasons for the collision

Survivors of the Southall crash described scenes of "carnage" last night as a public inquiry was promised into the cause of an accident that should have been impossible.

Passengers on the 10.32am Great Western Swansea-Paddington service had to dodge live electricity cables and climb over bodies. But Railtrack bosses confirmed that a "fail-safe" system was on trial on that stretch of track. They would not say, however, whether it was switched on.

The accident happened at 1.15pm a quarter of a mile east of Southall station in west London. The passenger train clipped the eighth of the goods train's 20 wagons, derailing at least four packed carriages and crushing many of those inside.

The driver of the passenger train was last night arrested in connection with manslaughter charges, British Transport Police announced. The development came as the last bodies were removed from the mangled wreckage at 9.50pm. Earlier, the driver of the goods train was released after being questioned by detectives.

Trials had been under way of the Automatic Train Protection system, recommended in the official report into the Clapham rail disaster in 1988, in which 35 people died. The system is supposed to make it impossible for a driver to ignore a signal set at danger, prompting speculation that the system was not in operation.

Railtrack's Chief Executive, John Edmonds, maintained: "We are content professionally that we have a proper system of maintenance. But clearly there has been a major fault somewhere. It's conceivable there was a technical failure and it's also conceivable there was a human error." Last night the Health and Safety Executive took the unusual step of announcing a full public inquiry to run parallel with the Railways Inspectorate's own.

Some of those on board were journalists returning from the vote on Welsh devolution. One, Nick Sutton, a BBC researcher, said: "As I walked off the train, I saw a body lying by the side of the tracks. No one was touching it. His shirt was ripped and there was blood all over him ... Everyone was shocked. There was a really strong smell. I don't know if it was the brakes or if it was from hitting a goods train."

Unconfirmed reports said the train braked hard at between 60mph and 90mph after passing through a green light as the goods train was crossing at an acute angle on to another section of track. Both drivers escaped without injury; 16 passengers remained trapped for two hours. Last night 13 people were described as seriously injured but only a few others remained in hospital. Nevertheless, the death-toll made it the worst rail crash since Clapham.

John Prescott, Deputy Prime Minister, promised a full inquiry. Questions will centre on the most likely causes: driver error, faulty signals or train derailment. Mr Prescott visited the scene. "It's terrible. Horrific," he said. The Health and Safety Executive inquiry was being made public, in line with Labour's policy of more openness.

David Eves, the executive's deputy director-general, said: "Our investigation into this collision began immediately. Railway inspectors are making detailed inquiries into the technical causes of the accident and into the actions of railway staff immediately before the collision."

Andy Hancock, acting director of Railtrack's south-western region, said the goods train was travelling on a relief line towards Southall yard. The collision took place on a set of points, at Southall East Junction, as the goods train crossed the track to enter the yard.

The London Ambulance Service sent 15 ambulances and 15 other transporters to the scene. Injured people were taken to Ealing Hospital, Central Middlesex Hospital, West Middlesex Hospital and Hillingdon Hospital.

One woman suffered spinal injuries and a fractured hip, and a man received arm, leg and spinal injuries. Another man with head and chest injuries was taken by air ambulance to the Royal London Hospital, Whitechapel.

Tony Mair, one of the first people to arrive at the scene, said: "We saw sparks and then there was a very loud bang and a ball of smoke. It was like very loud fireworks. I was there in under a minute. The train was lying on its side and people were wandering outside, with blood pouring from their faces.

"Two police officers were trying to warn us about [the possibility of] fallen power lines, but we were just worried about getting to help the people. There was nothing we could do to help people in the second and third carriages. My first impression was there were four bodies on the track."

Scotland Yard issued an emergency number for relatives last night - they should call: 0171-834 7777.

Further reports and pictures, page 3