Then she saw one of her brothers. In that era before smartphones, he was the one who broke the news that was hours old but unknown to her: Dennis, 52, her husband of nearly 27 years, was dead. Kevin, 26, had been shot in the head and left to fight for his life.

Father and son both worked for Prudential Securities, a financial services firm in Manhattan. They were heading home together that evening on the 5:33 Long Island Rail Road train out of Pennsylvania Station. As the train approached the Merillon Avenue Station, one stop from Mineola, a Jamaican immigrant on board, Colin Ferguson, reached into a bag, pulled out a 9 mm semiautomatic gun with a 15-round magazine and began firing. After emptying one ammo clip, he inserted another and emptied it, too. Finally, as he tried to reload once more, three passengers overpowered him.

The devastation was thorough. Five people in addition to Dennis McCarthy were dead. Eighteen were wounded besides Kevin McCarthy, whom hospital doctors gave slim odds of survival.

On learning what happened, Carolyn McCarthy slumped to her front steps. “I just started screaming a little bit,” she recalled in an interview for this obituary in 2014.

“And then,” she continued, “I said, ‘OK, let’s get to the hospital.’”

The resolve that she so quickly displayed would become familiar to — and widely admired by — people across Long Island and much of the country. The woman who rose from those front steps was soon to turn herself into a determined advocate for measures to rein in the country’s horrific gun violence.

But tending to her stricken son came first. She refused to accept that he would not live. She had been a licensed practical nurse for 30 years, working in emergency rooms and on many hopeless cases. “I took care of patients when nobody else would take care of them,” she said.

She was right about Kevin. Against steep odds, he pulled through and returned to work, albeit paralyzed on his left side, unable to perform simple tasks such as lacing his shoes. Velcro tabs kept them bound.

In short order, Ms. McCarthy threw herself into gun-control advocacy, though not at first with any thought of running for elective office. “All of a sudden, I had a voice,” she recalled in 2014.

Allies in the struggle for gun laws described her as tireless.

“She would, on the shortest of notice, devote herself to do a press conference, do an interview,” said Richard M. Aborn, president of the Citizens Crime Commission of New York City. In the mid-1990s, he was president of Handgun Control Inc., now called the Brady Campaign to Prevent Gun Violence. He added: “She never said, ‘Why?’ She said, ‘Where?’ and ‘When?’”

Ms. McCarthy also emerged as a spokesperson for Ferguson’s victims at his trial, in which his lawyers initially tried to argue that he had been driven temporarily insane by “Black rage,” a condition induced, they said, by racial injustice. Ferguson insisted on running his own defense, which involved ranting about supposed conspiracies against him and hectoring his victims when they testified. In February 1995, he was convicted of multiple counts of murder and attempted murder and sentenced to 315 years in a state prison.

With that, Ms. McCarthy was done with him. “You will be gone from my thoughts forever,” she said to her husband’s killer at a presentencing hearing. “And we will learn to laugh again. But you will not.”

The Long Island Rail Road massacre — Ms. McCarthy never called it that; for her, it was always “the incident” — helped lead to a 1994 federal law banning 19 kinds of semiautomatic weapons and the type of high-capacity magazine that gave Ferguson more time to keep shooting without having to reload.

That law was allowed to expire in 2004. But congressional Republicans tried to overturn it well before then. One of them was Representative Daniel Frisa, from New York’s Fourth Congressional District, in Nassau County, which was Ms. McCarthy’s district. Frisa’s vote in 1996 to repeal the so-called assault-weapons ban infuriated her. Someone had to take him on, she said.

And so she did.

When she first ran for Congress, Ms. McCarthy turned reflexively to Long Island’s Republican leaders, but they had no stomach for a divisive primary. Democrats were interested, though, among them Richard Gephardt, the House minority leader. Ms. McCarthy was so unfamiliar with Washington back then that she later recounted how “a guy named Dick Gephardt called me — didn’t know who he was.”

On a May morning in 1996, she formally announced that she would run against Frisa on the Democratic line (although she remained a registered Republican until 2003). With Kevin at her side, she faced news gatherers in front of her Mineola house. This was where she had lived since 1952, when her parents, Thomas and Irene Cook, moved from Brooklyn with their five children, including 8-year-old Carolyn. It was where she and Dennis were married, where she held a funeral reception for him and where she remained until 2015, when she moved to Florida.

She had a prepared speech that morning, but she tossed it aside after a fumbling attempt to read it. This was not theatrics. She had dyslexia, a condition that she did not discuss publicly at the time but would do so often in later years.

In the 1996 campaign, Ms. McCarthy became a popular presence: a political rookie at age 52, with hair tied in the back, bangs covering her forehead, a manner anything but slick and an accessibility as apparent as the flaming-red jacket that became her signature garment. Frisa waved her off as a Janie One Note, focused solely on guns. She didn’t care.

“Everyone that ever ran against me always said, ‘She’s only a one-issue candidate,’” she said in 2014. “I said, ‘Yeah, but at least I’ve got an issue.’ And by the way, I’m a woman. There’s no such thing as a woman having one issue.”

That November, she won convincingly, 57 percent to 41 percent. Triumph turned her into a national figure. NBC made a 1998 movie about her odyssey from nurse to a member of Congress: “The Long Island Incident.”

McCarthy would be reelected eight times, on occasion by slender margins but for the most part handily enough.

Winning elections, though, proved to be the easy part. Much harder was passing the laws she wanted. She learned that lesson right away.

For instance, she sought to have child-safety locks required for all handguns. “It is a simple safety lock,” she pleaded to the full House in 1997. “We have bills that make it impossible for children to get into an aspirin bottle. Do my colleagues not think we should do the same thing with a gun?”

But leaders of both parties, notably in the Republican majority, refused to attach her proposal to a juvenile-justice bill. Ms. McCarthy did not disguise her frustration.

“You couldn’t talk to me for two days,” she said in June 1997. She had learned firsthand the power of the National Rifle Association. “There are a lot of members who are just afraid to take that vote,” she said.

Time and again, the outcome would be the same. Whether the issue was restoring an assault-weapons ban after the 1994 law expired, or limiting the capacity of ammunition magazines, or requiring parents to store guns safely at home, or opposing legislation that shielded firearms manufacturers and dealers from liability lawsuits, Ms. McCarthy landed on the losing side. During all but four of her 18 years in the House, Republicans were in control, averse to gun regulation in almost any form. Some Democrats were no different.

She allowed that the repeated defeats could get her down. She also received death threats, some deemed serious enough for her to have police protection at public appearances. But, she said in 2014, “I never gave up.”

Peter T. King, a former Republican member of Congress from Long Island, said that as the years passed, Ms. McCarthy came to accept Washington’s realities. “She didn’t accept the result,” he said, “but she accepted the fact that it was going to be a long, hard fight.”

She did have one notable success: legislation to improve the system of background checks on potentially ineligible gun buyers, such as felons and the mentally ill. In January 2008, President George W. Bush signed this McCarthy-sponsored bill into law.

Other issues also mattered to her, including improving health care for the young and the elderly, fortifying environmental protections for Long Island Sound, and providing federal funds for the early detection of dyslexia.

But she did not delude herself about what accounted for her prominence. “I will never walk away from being called the gun lady,” she said in 2014, the year she announced she would not seek a 10th term. By then, having turned 70 and recovering from cancer, she decided that someone younger should be the go-to person for news reporters and others looking for comment as the nation’s gun horrors continued unabated, a grim litany of place names: Columbine, Aurora, Virginia Tech, Sandy Hook.

Sometimes, the carnage overwhelmed her.

“I think the one that hit me the most personally was Gabby Giffords, when she was shot,” she said in the 2014 interview, referring to Gabrielle Giffords, the Arizona member of Congress who was shot in the head in 2011 during a public appearance in Tucson but survived.

“She was shot just like Kevin,” Ms. McCarthy said. “I probably never stopped crying for three days, because it brought back so many memories of what Kevin went through, what we went through, what the other victims went through.

“And I said, ‘Surely, one of our own, they’re going to do something.’ No. Nothing.”

This article originally appeared in The New York Times.