An off-duty policeman saw a high-speed train hit a car on a level crossing, killing seven people in Berkshire.
All eight carriages left the tracks in the accident
He had contacted emergency services to say the car was stopped on the crossing, but did not have time to avert the tragedy, police said.
The 1735 First Great Western Paddington to Plymouth service was derailed in Saturday's crash, near Ufton Nervet.
Police have refused to discuss whether the car driver, who died in the crash, had been attempting suicide.
The seventh death was announced by Royal Berkshire Hospital on Sunday evening.
Dep Chief Constable Andy Trotter of British Transport Police said the officer was driving along the lane when he saw a saloon car stationary on the crossing with the barriers up.
"The barrier then came down and the police officer went to the emergency telephone, but before he could get a response the train came through and hit the car on the crossing," he said.
The officer only had a matter of seconds to act and had done "extremely well in the circumstances", Mr Trotter said.
He added that it was "quite remarkable so many people managed to escape from such an awful event".
Scenes of crime officers combed the site on Sunday alongside a pathologist and a coroner.
Mr Trotter said: "This will be a meticulous investigation to make sure we can find out exactly what happened and what caused the deaths of those six people."
Police are bringing in two cranes to help build a more powerful crane capable of removing the train carriages from the site.
Survivors have described the chaotic scene as the carriages and two engines left the track which has a 100mph speed limit.
Jon Stace, 21, a graphic design student from London, suffered deep lacerations to his arm, hand and back said he felt lucky to be alive.
FATAL RAIL CRASHES
7 July 2003 - three died when a First Great Western train hit a minibus on a level crossing
12 December 1988 - 35 died in a collision involving three trains
22 May 1915 - 200 died when a wooden troop train and a passenger train collided
He said: "I thought I was going to die. We couldn't see anything and all I felt was my body being dragged outside the window."
A total of 71 injured were taken to the Royal Berkshire Hospital in Reading and the North Hampshire Hospital in Basingstoke.
Scores more were treated at the scene.
A spokeswoman for the Royal Berkshire Hospital said a man who had been in a critical condition died on Sunday.
"He was the only one of our patients who was in a life-threatening position.
"We have got a remaining seven patients with minor injuries and four more serious but stable. They are not life-threatening," she added.
The Queen has sent a message of sympathy to the victims' families and friends and Transport Secretary Alistair Darling has promised an investigation.
Britain's main passenger group has called for a thorough inquiry into the safety of level crossings.
Rail Passengers Council national director Anthony Smith said: "There are two big questions which need answering quickly - what was the car doing on the railway line and why did the train derail in such a catastrophic fashion?"
The level crossing was unmanned with half barriers on each side of the road.
The car driver was killed
Earlier this year, Her Majesty's Railway Inspectorate said level crossings posed the "greatest" potential rail risk.
It said existing level crossings were gradually being removed, with no new ones being built.
But railway expert Christian Wolmar said it would be unrealistic to call for them all to be scrapped.
People worried about friends or relatives should call the Thames Valley Police casualty bureau number on 0870 010 0732.