By Jonathan Duffy BBC News Online, Potters Bar

The ear-splitting screech of several tonnes of grinding metal brought lunchtime shoppers in Potters Bar to a sudden halt on Friday.

Then came the crash of the railway bridge that bisects the centre of this commuter town, as it crumbled onto the road below.

Shoppers like Mary Kinsella and Maysie Bowen, who had just wandered across the station forecourt between shops, stayed rooted to the spot.

"I stood here and was absolutely flabbergasted," said Miss Kinsella.

Mary Kinsella (right) and Maysie Bowen

"There was a bang and then an explosion and when I looked round there was black smoke and rubble everywhere."

She is from nearby Hatfield, itself notorious for being the scene of a rail crash in October 2000, in which four people died.

As yet, it is unclear what caused the 1245 service from London's King's Cross to jump the rails and break up, with one carriage flipping onto the station platform.

Rush to help

After the initial shock the shops emptied as people ran to offer help.

"I found a woman lying under the bridge," said the owner of Sunshine Hairdressers whose premises are in a small row of shops backing onto the railway.

"She was not in a good way. Then ambulance people came and took care of her."

Travel agent Chris Turner had been sitting in the back of his shop having lunch when the crash happened.

Chris Turner: "The building shook and the lights flickered"

"The train sounded like a goods train coming through - much heavier and louder than a passenger train," he said.

"Then suddenly there was a horrible noise.

"The building shook and the lights flickered and it was a while before I realised what had happened."

Caught up in the terror of the drama were pensioners Ron and Gill Tarling who had been driving under the bridge when it started to collapse.

"We were in a traffic jam and heard the train coming," said Mrs Tarling, who was at the wheel of their Vauxhall Corsa.

"When you're under a bridge like that, the sound is always amplified but it just kept getting louder and louder.

Raining rubble

"Then there was a grinding noise and suddenly stuff was raining down on our car.

"It must have been masonry from the bridge but we didn't know it at the time."

Lucky escape: Ron Tarling was slightly injured

Mrs Tarling's involuntary reaction was to grip the steering wheel as hard as she could.

Her husband covered his head with his hands as the front and back windscreens caved in around him.

"I didn't know what was going on or what was going to happen next," Mr Tarling said.

"It felt like it went on and on but probably it was all over in a couple of minutes."

It wasn't until the couple got out of the car that Mr Tarling noticed blood on his hand.

"It looked worse than it was," he said. "I got a cut on my head from some glass that had shattered in the car."

He was escorted to the nearby Sainsbury's where about 25 other slightly wounded casualties were being treated by staff and a nurse who had been shopping in the town centre.

Lucky escape

The Tarlings, who were on their way from their home in nearby Wheathampstead, to a family reunion, said they were lucky to get off so lightly, especially as the roof of their car had collapsed.

The people of Potters Bar were clearly aghast that such a tragedy had happened on their doorstep.

As blue lights flickered on top of the emergency vehicles which crowded the centre of the town, shopkeepers could only stand and watch the recovery operation in progress.

One local woman delivered a bunch of pink carnations across to a police officer keeping watch at one of the temporary cordons.

A waitress from Jayne's Place cafe stared blankly at the activities outside.

"All I can think of is thank God it didn't happen a couple of hours later when that platform is packed with children coming home from school," she said.