(CNN) -
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 she would be late.
He found an abandoned baby on the subway and called 911 from a pay phone.
Stewart, a social worker, had seen a small bundle wrapped in a sweatshirt while walking through an eerily empty station.
At first, he thought it was a doll, perhaps abandoned by a child, until he saw a small leg move.
He quickly discovered that it was a newborn baby, with the umbilical cord still attached.
Mercurio, the author of a book about this chance encounter, spoke to CNN about that night and how he and Stewart ended up raising the baby as their own.
The baby was only a few hours old when Stewart found him
"In fact, he had tried to get on an express train and couldn't do it," Mercurio recalls of Stewart's road that day.
"The fact that he even got on a local was something miraculous because who knows if he had gotten on an express, if he would have found the baby."
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Mercury says something made Stewart look at the lump again and see the tiny movement of the newborn.
The baby, a boy, was alive and breathing.
Authorities said he was only a few hours old when they arrived.
Mercurio ran to the station, one block from his apartment, and found Stewart there with two policemen.
"One of them was carrying the baby in his arms," he says.
"Just a chill went down my spine. It's unbelievable."
The baby was taken to a nearby hospital and the men were overwhelmed with emotion over what had just happened.
"At one point I turned to Danny and said, 'You're going to be, we're going to be, connected to this baby in one 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 will find out tonight and maybe he wants to try and find you and maybe we can send a gift or get in touch with him at this day, every year. '
The couple had no idea what was about to happen.
A stroke of good luck made the adoption process easier
The baby, who was named Daniel Ace Doe in honor of Stewart and the A / C / E subway line, was in the care of the state while a city-wide search was conducted 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 2000, at that hearing, the judge asked him: 'Would you be interested in adopting?'
He said, 'Yeah, but I know it's not that easy.'
She said, 'Well, it can be.
We didn't know what he meant by that, "says Mercurio.
The two became adoptive parents to the baby, whom they named Kevin, and adopted him two years later, in December 2002.
"And that was it. We never saw the judge again. And in 2012, when we decided to get married, Kevin, while walking to school, said he knew there was a judge involved in creating our family," says Mercurio.
"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 who finalized your adoption?'
And he nodded. And that's how I got in touch with the judge again, ten years later. "
When the couple spoke with the judge, they asked how she was able to help facilitate Kevin's adoption so quickly.
In one of the many little miracles that Kevin was brought to, the judge said that at the time Kevin was found there was a pilot program in New York that gave him the authority to speed up the adoption process in specific cases of abandonment for 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," says Mercurio.
"So it didn't languish in the system."
That pilot program lasted only six months and was later discontinued, according to Mercurio.
"So many little things like that added up without our knowledge," says Mercurio.
Kevin is now a senior in college.
He is not sure about trying to find his biological parents.
When Kevin arrived, the couple did not have much money.
They were in debt for student loans, but they made it work.
Family and friends gave them everything they needed for Kevin, and they sorted it out along the way.
Kevin, now 21, is a student at Swarthmore College.
Mercury is sharing his family's story with the support of Kevin and Stewart, but they declined to be interviewed.
"We still can't believe it. I mean, we believe it because we have a 21-year-old who will graduate from college this spring," says Mercurio.
"I love this boy 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."
When Kevin grew up, the two of them talked to him about their family history.
"We talked openly about how our family became a family in front of him. When he attended social gatherings, [if] someone asked, we didn't protect him from hearing it from a very young age," Mercurio says.
They wanted Kevin to feel positive about his family's origin story, so Mercurio wrote a book about it that they read to him every night.
When he was five years old, Kevin realized that it was about him.
"I pasted a book on his story, which tells everything about Danny being on the subway and they found the baby," says Mercurio.
Last year, that very personal story was published under the title "Our Subway Baby," which Mercury calls a "love letter to our son."
While Kevin's biological parents are still unknown, Mercury says they only feel compassion for them.
"One way or another, that's a desperate measure. And I can only imagine the anguish of the person who was leaving his son," he says.
"We've always told Kevin from a young age that they left him out of love, so they could find him and take care of him. We never used the word abandoned or abandoned. We said 'she left you where we could find you.'