A first-aider has described how he rushed to help passengers caught up in the Potters Bar rail disaster.

Computer consultant Brian Sadler, of Kingston Lisle, near Wantage, had to make his way under part of the train and across tracks to check on the dead and seriously injured.

He tended to wounded for about an hour before leaving them in the hands of paramedics.

The trained Red Cross member, 59, said: "It was a scene of devastation. It was horrific.

"I'm a bit traumatised and I didn't sleep very well. There are images I can't get out of my head."

Mr Sadler works for Alchemy Data Solutions, which is on the fourth floor of the building directly above Potters Bar station in Hertfordshire. He was on his way to lunch when he heard a loud bang and immediately realised it was a serious incident.

He made his way across a platform and under the carriage -- which straddled two platforms and was wedged under a canopy -- to check on casualties.

"Potentially the carriage could have fallen towards us, but I think it was firmly wedged under the canopy," he said.

He saw three casualties on one platform and four or five lying on the tracks. Two victims on the tracks were dead, including a young woman.

After assessing the injured, he made his way back to tend to two people on one of the platforms.

"One of those was a man, aged about 30, with chest injuries. I think he may have injured his neck," he said.

Seven people were killed after one of the carriages broke free from the rest of the Kings Cross-Kings Lynn train.