A policeman waiting on Potters Bar station platform smashed the window of a derailed train with a chunk of concrete to get to a trapped woman.
Pc David Bedford told an inquest he was waiting to catch a train to work when he heard a massive bang, then saw the train emerge from a cloud of dust.
He said he first "ran for his life", but when the train came to a halt, jumped on to the track to try to help.
Seven people died in the crash at Potters Bar station on 10 May 2002.
Pc Bedford was speaking at the third day of an inquest into the crash in Hertfordshire.
He said he was on his way to Wood Green police station in north London, when the 1245 train from London King's Cross to King's Lynn in Norfolk derailed and the last of four carriages became detached.
Pc Bedford said: "I had my CD player on. I was listening through headphones.
"I heard a noise which sounded like a very large explosion behind me. I saw a large plume of dust or smoke swirling in the air. From out of this emerged a train travelling north.
"I noticed it had only three carriages as it emerged from the dust and debris and behind it I saw the fourth carriage which seemed to turn sideways then tip over. And it kept sliding forwards."
He added: "I saw it flip over once. It took a second to register what I had actually seen.
"At that point I turned and ran. I thought it was going to hit me and other people on the platform."
Pc Bedford said after the carriage came to a halt he jumped on to the track.
Covering the dead
He said he saw a man lying on the track and people lying under the carriage. Pc Bedford said he saw two "Oriental-looking females" dead on the track.
He said he asked a rail worker to find blankets to cover the dead.
Pc Bedford said he then used a "football-size" chunk of concrete to smash a window so he could get into the carriage with a fireman.
He said he found an injured woman called "Maria".
"I simply sat there with her head in my lap and talked to her," Pc Bedford told the inquest jury.
He added: "I sat with Maria until paramedics arrived."
Pc Bedford choked as he told the court of the terrible injuries he witnessed - and how he observed a number of bodies on the track.
He described how he exchanged words with one male passenger who had been flung from the carriage.
"I spoke to him to ask him if he was OK. He blinked and said he was. I was relieved that he was alive," he said.
As well as breaking the window of the carriage to help those inside escape, the police officer went on to help paramedics stretcher people out.
Lecturer Louis Spring, another passenger who was waiting for a train, described hearing an "almighty noise" as he emerged from a waiting room at Potters Bar at the time of the crash.
"There was a loud crashing... it felt like things were breaking up," he said. "Dust was filling the air and there was debris everywhere."
Mr Spring said he heard screams for help and had jumped down on to the track to try to assist.
One woman was shouting "get me out, get me out", he told the court.
He also recalled seeing horrific injuries and said he and two others tried to reassure those passengers who were still conscious.
Mr Spring, who told the court of the "sense of guilt" he still feels for not being able to do more that day, described how he had tried to bring comfort to one injured woman.
"All I could do was pray. I had my arms on her and prayed," he said.
Sally Hatton, a hairdresser working at Sunshine Hairdressers in Potters Bar, described in a statement how she discovered 80-year-old victim Agnes Quinlivan trapped under rubble beneath a railway bridge.
She said she stayed with her "stroking her hair and hand" until paramedics arrived.
Ms Hatton then went to the station and helped other injured passengers.
Workers from a nearby Sainsbury's also told the court how they took first aid kits from the store to help the injured at the station. Other staff also assisted injured passengers at the supermarket, which became a make-shift hospital.
The West Anglia Great Northern train careered off the track at 98mph, flipping its rear carriage into the air.
It crashed into a bridge and slid along the platform before coming to rest under the platform canopy. Six passengers in the rear carriage and a passer-by were killed.
The inquest will examine the deaths of Austen Kark, Emma Knights, Jonael Schickler, Alexander Ogunwusi, Chia Hsin Lin, Chia Chin Wu and Agnes Quinlivan.