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The powerful lure of smartphones has created a heads-down culture in many public places. John Mervin in New York came across someone who just might benefit from a little digital detox.

I'd never saved someone's life before, so I wasn't sure of the protocol.

Speechless incomprehension on my part didn't seem appropriate. But then neither did the young woman's giddy laughter, or her jaunty departure from what could so easily have been the scene of her death.

It took my daughter's torrent of questions, as we turned away, to force the world back into a semblance of order:

"Daddy - what was she doing? Why was she on the tracks? What would have happened if a train came?"

As we climbed the stairs into New York's gleaming winter sunshine, I tried to explain what we had just seen.

We'd arrived at Rockefeller Center station on the D train. As in many of New York's underground stations, trains pull in at both sides of the platform. Or, rather, they seem to erupt into the station first on one side, then on the other.

When the stations are busy, the platforms feel like narrow, crowded islands of safety. We picked our way along this one, my wife and youngest daughter in front, my eldest daughter and I at the rear.

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Abruptly, my wife stopped.

"Uh, what's this?" she said.

I looked over her shoulder. There at our feet lay a young woman of about 20. She was on her stomach with the top half of her body on the platform, while her legs dangled over the tracks kicking pathetically.

She was stuck. She had also, clearly, been down on the tracks.

And just as each commuter imagines, as they stand on the platform edge pondering the end of it all, she had discovered that climbing back up from the tracks is really hard.

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The lip of the platform sticks out so far that you have to climb out as well as up. That leaves you straining to keep half your body on the platform while the other half flails wildly for some purchase in mid-air.

But unlike in our morbid imaginings, this woman was not in the grips of panic, anticipating her imminent decapitation by the F train which would be screeching into the station in the next few minutes, if not seconds.

She was laughing! Giggling! So was her friend who half-heartedly leant down to assist.

The assistance was somewhat compromised by the fact that the friend was holding her mobile phone. Was she hoping to capture this moment with a picture? Or composing a text?

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It's well known that people's compulsive checking of their phones can be deadly. Among young people in America, texting is now the number one cause of car crashes.

Maybe it's also a leading cause of leaving friends to perish when they fall in the river or on to the train tracks.

In any case, I stepped forward.

Absurd as it might seem, my immediate concern was which part of her body it was OK to touch.

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For the mechanics of dragging her to safety the obvious place to grab would have been her inner thigh. But that seemed indecent. An assault even.

Well, what about the belt loop on the back of her jeans? No! That would wrench her clothing into some painful, awkward position.

But for goodness sake, she was about to be killed. This wasn't the time to fret about the niceties! So I leant out as far as I could, got hold of her leg somewhere near the knee and, together with her finally-engaged friend, hauled the young woman on to the platform.

New York's transit authority constantly warns passengers not to go on to the tracks for any reason. But there is a constant stream of stories of people who have done so and been hit, and crushed, by trains, or of people who have fallen and then been struck. There are even a few stories of miraculous escapes.

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In 2015, 50 people were hit and killed by subway trains here. Not that many in a city of eight million, but enough so that for all their drab functionality the stations have an air of profound, nascent danger.

The islands of safety are surrounded by a lethal void.

Though maybe it doesn't seem that way to someone still young enough to be fearless. The woman I helped did get out alive.

And you can guess why she'd been on the tracks. Still laughing, but maybe chastened by my look of horror she said: "Thanks. Sorry. My phone fell down there. It would have taken them forever to get it back."

While I turned to clutch my daughter's hand and head upstairs, the young woman and her friend sauntered away. I wonder when she'll be scared?

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