By the time the officers entered the subway car, they could see the baby's head crowning through his mother's sweatpants.
A group of riders had already formed a semicircle around her, offering as much protection and comfort as they could on the Market-Frankford Line.
SEPTA Police Sgt. Daniel Caban and Officer Darrell James arrived at the 15th Street station about the same time: 5:50 p.m. on Christmas.
"Get your gloves ready," Caban told James.
Caban, who had experienced childbirth only as an observant father, knelt and removed the woman's sweatpants as she practiced breathing exercises.
"I knew I had other officers around me, so I was good," Caban said in retelling the story with James an hour later.
The baby soon landed in his arms. It was a boy, and although the umbilical cord was slightly tangled around his neck, he appeared healthy.
There was another challenge, though. No one in the car could communicate with the boy's mother and father, an Asian couple in their late 20s or early 30s who apparently spoke little English.
"Are you Dad?" they said to the man. He nodded. They motioned for his shirt. At first he offered his beat-up tank top but then gave the officers a light-colored sweater, which soon swaddled the newborn.
"Do you want to hold the baby?" they asked the mother.
She extended her arms.
Onlookers at the 15th Street stop knocked on the windows and gave a thumbs-up to everyone in the subway car.
"The mother was at peace with the baby," Caban said.
A minute later, the medics arrived, and the family was off to Hahnemann University Hospital.
Caban and James, who have families of their own, were hoping for a quiet Christmas night. But they felt blessed by the experience.
"It was the best Christmas gift that I would ever be able to ask for," James said. "Having a kid born on Christmas but also being able to help a family."