The off-duty officer had been driving down a country lane near the village of Ufton Nervet when he came across a saloon car parked on the level crossing. He watched as the warning lights flashed and the barriers came down with the vehicle still stationary on the tracks. The policeman jumped from his car, screaming at the male motorist to run away. The young man is understood to have shouted back that he wanted to die.

The officer, who has not been named, is then reported to have rattled the door handles of the locked grey- coloured car, before running to the emergency telephone beside the railway track in a vain attempt to stop the train. Seconds later the 17.35 London Paddington to Plymouth First Great Western express loomed out of the fog and smashed into the car at 100mph, killing the motorist and six others on the train, including an eight-year-old girl. It emerged last night that the train ’s driver had suffocated in his cab after it flipped on to its side and scooped up tonnes of mud and shingle. His body had to be dug out.

More than 30 people were seriously injured and ambulances from Berkshire, Hampshire, Surrey, Oxfordshire and London attended the scene.The police officer is receiving counselling after witnessing the accident at such close quarters.

Police are treating the incident as suicide but said that they have not ruled out other explanations. “It looks like he deliberately went on the tracks and stayed there,” a police source at the scene said. “It has the hallmarks of suicide but we are keeping our options open.”

He said that he could not completely rule out the possibility of mechanical failure. But this would not explain why the motorist did not leave his vehicle.

Senior officers spent yesterday debriefing the policeman at a local station. He will make a second statement today when investigating officers will be looking at what motive the motorist could have had for committing suicide.

Deputy Chief Constable Andy Trotter, of British Transport Police, said: “The officer did absolutely the right thing at the scene. He went to the emergency phone in order to alert the signals. It all happened in a very, very short space of time.

“I can’t imagine what it was like for that officer, witnessing the scene. I have been down there today and it is a very difficult scene to take in.”

Mr Trotter last night confirmed that the officer had witnessed the crash at close hand. He said: “We do know that an off-duty Thames Valley police officer saw the car go on to the level crossing. At that time the barriers were up. The barriers then came down and the police officer tried to use the emergency phone but before he could get any response, the train came through and hit the car.”

He refused to discuss further details of what happened, although he confirmed that the police knew the identity of the motorist.

One of the first people to reach the crash scene spoke of the horrors he saw on the tracks and inside the mangled carriages.

Mark Penston, a father of two who was on his way home from a fireworks display with his family, said: “There was a body by the level crossing. I think it was out of the car. I didn’t really want our kids to see it.

“The thing that struck me initially was the silence, no one crying, screaming or shouting. It was total darkness apart from light sticks. People were absolutely confused and no one knew what had happened. There were people wandering about.”

After directing several people away from the crash site, Mr Penston, an independent financial adviser, clambered into one of the carriages that was lying on its side. He said: “There were two people trapped in it. A doctor was there and said one of them was dead. The live person was trapped beneath the dead person. It was a nasty situation.”

After the accident, the Mazda 323 lay split in half, scattered among the debris of the derailed carriages.

Police teams, with specialist search dogs, and dozens of firefighters were last night still working around the wreckage, picking their way carefully through the sharply twisted metal.

One half of the shattered car was covered with a yellow tent, as a forensic science unit worked nearby, trying to piece together what happened.

The warning sign before the level crossing, which reads “Stop when the lights are red”, was still flashing yesterday morning.

Keith Lock, a local district councillor, said: “If it was a suicide, it is wicked for somebody to have ended their life that way.

“Everyone knows the trains go through here at 100mph. It’s terrible.”

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