A woman in the US city of Philadelphia got an unexpected holiday surprise on the subway when she went into labour, delivering a healthy child dubbed the "Christmas baby".

The woman was on a train on Thursday in central Philadelphia in the US state of Pennsylvania when her water broke.

Two transit police officers were urgently called to the train as the woman went into labour.
The officers helped the woman deliver as fellow passengers looked on, and one transit worker was photographed leaving the train with the baby wrapped in clothes.

Sargent Daniel Caban said he was hoping for a quiet day on the job, but the delivery was a "pleasant" surprise.

"I was just praying for the medics to come in and take over," he told an NBC station.

"This was just a blessing, during the holidays, a Christmas baby."

His colleague, Darrell James, said the birth made for a great Christmas.

"Everything just happened so quick but it was amazing.... the mother wanted to hold the baby, the father offered the shirt off his back to warm the baby up," he said.

"This is the best Christmas I ever had."

The mother and child were taken to hospital and are doing well.

The officers have also recovered from the emotional experience.

Southeastern Pennsylvania Transportation Authority chief Thomas J. Nestel III applauded the officers, posting a picture of the two men on Twitter.

"WOOHOO Now that's a Christmas gift" he tweeted after the delivery.

He said the child, the subway's newest rider, was not charged a fare.