Who knows what great reserves of courage and strength may exist within us, waiting for those extraordinary moments when we need them most? One who knows is Carolyn McCarthy - a remarkable woman whose extraordinary season in hell transformed her forever.

Her ordeal began Dec. 7, 1993, when McCarthy heard the news that a man had gone berserk on a busy commuter train in New York's Penn Station and started shooting passengers with a semiautomatic handgun. Her husband and son were on board that train.Of the six people killed by Colin Ferguson that day, one was McCarthy's husband. Of the 19 people wounded, one was McCarthy's son, whose prognosis was so poor that he was nearly pronounced brain-dead.

On that day, McCarthy was principally a wife and mother, though she had trained as a licensed practical nurse. Unprepared for the tragic events of that day, she somehow found the courage to face the grim facts: Her husband of 30 years was dead, but her 26-year-old son, Kevin, had a slim chance for survival. She would pour all her energy into helping him beat almost impossible odds.

McCarthy's painful, moving and inspiring story is told tonight at 8 p.m. on Ch. 5 in NBC's docudrama, "The Long Island Incident." In absorbing fashion, it charts her arduous campaign to bring her son back from horrendous brain injuries, then shows us how McCarthy took up the fight to stop such massacres from ever happening again - by becoming one of this country's most persuasive advocates for gun control. Her tireless efforts won her a seat in the U.S. Congress.

In a special session with TV critics, McCarthy explained why she was willing to revisit the worst nightmare of her life in the form of a made-for-TV movie.

"We're hoping that this particular story will give people the strength and courage to say: I can do it," says McCarthy. "And you can."

Sitting beside her on that day with the critics was her son, who has nearly recovered from his injuries - which at first left him paralyzed with little hope of ever walking again. He credits his mother with pulling him back.

"It's all inner strength," he says. "I don't know where she gets it from."

Played forcefully in the film by Laurie Metcalf, best known for her comic role as Roseanne Conner's sister in the long-running "Roseanne," McCarthy is portrayed as an average woman who forced herself to rise above anyone's expectations for her, including her own. It was a private and personal process, but the tremendous publicity about the massacre - and the murder trial that followed - dragged it into public view.

McCarthy's son, Kevin, is played by Mackenzie Astin and Colin Ferguson is played by Tyrone Benskin.

McCarthy at first balked at the idea of a movie about her ordeal.

"I'll be very honest with you," she says. "I didn't want the movie made at all, mainly because Kevin and I would like to bring some privacy back. But once I ran for Congress, that was out the door and into the public domain."

Still, McCarthy refused to cooperate with any film that would dramatize the massacre itself. She received many proposals - and all but one included plans for scenes aboard the commuter train. She turned those down, then read the script submitted by Barwood Films, the company run by superstar Barbra Streisand and Cis Corman. It proposed telling the story McCarthy felt ought to be told - and it would be directed by multiple Emmy-winner Joseph Sargent ("Miss Evers' Boys," etc.).

The movie that resulted is not a courtroom drama about the fight to make Ferguson pay for his acts of "black rage" against the mostly white and Asian passengers he targeted but about the hidden resources within the average person and how they can be tapped in times of great need. We watch as McCarthy molds herself into a public person to make a difference in the world.

Though a Republican, McCarthy quickly learned that her party was turning its back on the assault weapons ban then proposed by President Clinton. In order to campaign effectively against the Republican incumbent in her New York congressional district, who opposed the ban, she had to win the Democratic Party nomination and then prove that a novice in politics could defeat an entrenched incumbent. That's exactly what she did.

Once in office, McCarthy realized how much she had to learn about politics.

"You're really on your own," she says. "You're responsible for everything. Yeah, it took two or three hours a night of reading, but you know what? A normal person can go to Congress and learn how to do the job, just like our forefathers said."

Oddly, McCarthy remains a registered Republican, though she sits on the Democratic side of the house.

"I guess that upsets a lot of people," she says. "It doesn't upset me. I vote the way I want to vote."

Because a disturbed man was able to acquire a deadly handgun legally, then use it to commit mass murder, McCarthy was forced to confront her attitudes about gun control head-on. Before the massacre, she was concerned about the rising tide of gun violence but assumed "somebody" would do some-thing about it.

"Well, we didn't," she says, which is why she finally used her position as a much-publicized victim of gun violence to help pass new legislation. She now believes the legacy of the Long Island commuter train massacre may be a much greater public commitment to change, saying, "Today there's a much larger (number) of everyday citizens that are starting to get involved."