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.

    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 hung over the tracks kicking powerlessly.

    She was stuck. She had also, clearly, been down on the tracks and discovered that climbing back up is really hard.

But unlike in our imaginings, this woman was not in panic, expecting her approaching death by the F train which would be screaming into the station in the next few minutes, if not seconds.

    She was laughing! So was her friend who half-heartedly leant down to assist. The assistance was somewhat weakened by the fact that the friend was holding her smartphone. Was she hoping to capture this moment with a picture? Or composing a text?

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 die when they fall in the river or on to the train tracks.

    I stepped forward, leant out as far as I could, got hold of her leg somewhere near the knee and, together with her finally-engaged friend, dragged the young woman on to the platform.

    And you can guess why she'd been on the tracks. Still laughing, but maybe chastened (内疚）by my look of horror she said, “Thanks. Sorry. My phone fell down there. ”

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