A man found an abandoned baby in a subway. It led to an unexpected family and a beautiful children's story

A New York family who came together by chance 21 years ago has now shared their remarkable story in a children's book.

Pete Mercurio was walking out the door to meet his then-partner (now husband) Danny Stewart for dinner in August 2000 when his phone rang. It was Stewart, calling to tell him he'd be late. He'd found an abandoned baby in the subway and had called 911 from a payphone.

Stewart, a social worker, had spotted a little bundle wrapped in a sweatshirt while walking through an eerily empty station. At first, he thought it was a doll, perhaps left behind by a child, until he saw a tiny leg move. He quickly discovered it was a newborn baby, the umbilical cord still attached.

Mercurio, who authored a book about this chance encounter, spoke to CNN about that night, and about how he and Stewart ended up raising the baby as their own.

The baby boy was just hours old when Stewart found him

"He had actually tried to get on an express train and couldn't get on one," Mercurio recalls of Stewart's path that fateful day. "The fact that he even got on a local was kind of miraculous because who knows if he had gotten on an express if he'd even had found the baby."

Mercurio says something made Stewart glance back at the bundle and see the newborn's small motion. The baby, a boy, was alive and breathing. Authorities said he was just hours old when they arrived.

Mercurio ran to the station, a block away from his apartment, and found Stewart there with two police officers.

"One of them was carrying the baby in his arms," he says. "Just a chill raced up my spine. Like, it's an unbelievable thing."

The baby boy was transported to a nearby hospital, and the men were overcome with emotion at what had just occurred.

"At one point I just turned to Danny and I said, 'You're going to be -- we're going to be -- connected to this baby in some way or another for the rest of your life.' I said, 'Maybe not tomorrow or next week or a year or five years from now, but eventually he's gonna learn about this night and he might want to try to find you and maybe we can send a gift or be in touch with him on this day, every year.'"

The couple had no idea what was about to unfold.

A stroke of good luck smoothed the adoption process

The baby, who was named Daniel Ace Doe after Stewart and the A/C/E subway line, was in the state's care while a citywide search was underway for the boy's parents.

Three months later, Stewart was asked to testify at a hearing about the day the baby was found. The judge asked him an unexpected question.

"In December of 2000, at that hearing, the judge asked him, 'Would you be interested in adopting?' He said, 'Yes, but I know it's not that easy.' She said, 'Well, it can be.' We didn't know what she meant by that," Mercurio says.

The two became foster parents to the baby, who they named Kevin, and adopted him two years later in December of 2002.

"And then that was it. We never saw the judge again. And in 2012, when we were deciding to get married, Kevin on a walk to school said he knew that there was a judge involved in creating our family," Mercurio says.

"We shared everything with him, so he knew everything. He said, 'Don't judges marry people?' So, I said, 'Do you want to meet the judge that finalized your adoption?' And he nodded his head. And so that's how I got in touch with the judge again, ten years later."

When the couple spoke with the judge, they asked her how she was able to help facilitate Kevin's adoption so quickly.

In one of many small miracles that brought Kevin to them, the judge said that at the time Kevin was found there was a pilot program in New York that gave her the authority to expedite the adoption process in specific cases of abandonment to place a baby in a loving home.

"She was able to make quick decisions to place that baby in a pre-adoptive home as quickly as possible," Mercurio says. "So he didn't languish in the system."

That pilot program lasted only six months and was then discontinued, according to Mercurio.

"So many little things like that added up without our knowledge," Mercurio says.

At the time Kevin arrived, the couple didn't have a lot of money. They were in student-loan debt, but they made it work. Family and friends got them everything they needed for Kevin, and they figured it out along the way.

Kevin, now 21, is a student at Swarthmore College. Mercurio is sharing their family's story with Kevin and Stewart's encouragement, but they declined to be interviewed.

"We still can't believe it. I mean, we believe it because we have a 21-year-old kid that's graduating from college this spring," Mercurio says. "I love this kid more than anything in the world, I really didn't know this kind of love existed in this world until my son came into our life. And Danny feels the same way."

As Kevin grew up, the two discussed their family story with him.

"We talked about how our family became a family openly in front of him. When he went in social gatherings, [if] anybody would ask, we didn't shield him from hearing it from a very young age," Mercurio says.

They wanted Kevin to feel positive about their family origin story, so Mercurio wrote a book about it that they read to him every night. When he was five, Kevin realized it was about him.

"I pasted together a book of his story, which tells the whole thing about Danny being on the subway and the baby being found," Mercurio says.

Last year, that very personal story was published with the title "Our Subway Baby," which Mercurio calls a "love letter to our son."

While Kevin's biological parents are still unknown, Mercurio says they feel only compassion for them.

"One way or another, that's a desperate measure to take. And I can only imagine the anguish that was leaving your child," he says. "We've always told Kevin from a very young age that he was left out of love, so that he could be found and cared for. We never used the word abandonment or abandoned. We said she left you where you could be found by us."