A 13-year-old boy is in a critical condition after falling from a moving train while allegedly attempting to train surf in Sydney's south.

The teenager sustained serious injuries to his face, neck and arms after falling off the back of a moving train at Caringbah yesterday evening.

Train surfing is the act of riding on the outside of a moving train and is a trend that is often filmed by the culprits and posted to social media.

The line was closed for over an hour about 7pm yesterday as paramedics worked to stabilise the teenager.

The victim's friends rushed to the scene after hearing the news.

"All these ambulance crews came, they were cutting the fence, then they put him on a stretcher," said one of his friends.

Despite its dangerous nature, train surfing continues to soar in po

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