Margaret didn't see him slip into the seat behind her so she can't remember at what point she became aware of him, a slim, youthful presence in a dark blue windbreaker who could easily have been one of her students, or even a son. Like her, he was sitting riverside, his modest knapsack hanging off the empty seat rack in front of him.

He must have boarded at one of those little stops between Yonkers and Poughkeepsie because he was in his seat by the time they passed the faux 19th-century castle near Rhinebeck. She noticed him looking wistfully at its half-fallen battlements on the Hudson, the graceful, silver river that unfolds like a party streamer through the hills of upstate New York.

At dinner the night before, her son said the Hudson was full of PCBs. Luke mentioned the pollution when Margaret confided she was taking the train home. He admitted he no longer flew himself and pulled out the hiking boots he'd bought for himself and Tina, along with two enormous, waterproof knapsacks. Cars were useless in that situation, Luke said. Her son was a slight, red-headed man with a round-eyed, boyish manner, and he'd emphasized the word "that" with a sad little smile.

While Margaret ate the boeuf au jus, too polite to admit she no longer touched red meat, she imagined Luke and her daughter-in-law hurrying to put on their sturdy, made-in-Canada boots, stuffing their shiny knapsacks full of photographs -- the single household object that could never be replaced. She imagined plump, good-natured Tina weeping as Luke slipped the thick-soled shoes onto her feet.

Yes, she could see her son leading Tina up 92nd, through the crowds gathered around cars with their passenger doors flung open, the radios going. They would slow down to hear the latest news and then walk away breathlessly, their faces pointing north like retrievers. Tina and her soft-hearted boy. Throwing in the towel. Getting the hell out.
Coming back from the snack bar, Margaret noticed the young passenger sitting with his head in his hands. She wondered why he looked so weary; perhaps, like Luke, he worked at a job he no longer liked. Her son was still in his same old office and joked about coming down with the World Trade Center cough.

Margaret felt exhausted herself, although there was no point trying to sleep with all the racket in the club car. She was worn out from the rally at the Roseland Ballroom when thousands of her countrymen had turned up for the long day of speeches and prize-giving. Not a word about the Canadian rally in the newspapers, of course. "We aren't different enough to count," Luke said affably when he noticed Margaret searching in the Times for the story.

Yesterday evening, she and Luke and Tina had sat three around a table in her son's tiny apartment on the Upper East Side and tried to pass off the slight with rueful jokes about their countrymen whose polite reserve often drew condescension in New York.

As if a quiet manner meant you were dull. It was unlikely that any thinking person could confuse these two traits now.
Somewhere near Albany, the Amtrak attendant put a trainee in charge and flopped down in the seat across the aisle. Margaret watched as the attendant wrapped herself up in a huge floral comforter and closed her eyes. A few minutes later, the attendant began snoring noisily.

If only I could snatch an hour of rest, Margaret thought, although lordy, there were things she should be doing. She could write Luke and tell her boy what was in her heart. Margaret pulled out her laptop and began: "Darling Luke! Don't wait until it happens again! Take Tina and head out now!"

She paused. She shouldn't be so frank with Luke, but she hating mincing words. The freedom to speak your mind was one of the perks of literary life; she knew it from the writing she did and the workshops she taught. Of course, fiction didn't cure heartbreak; it described it and woe to those who didn't know the difference.

As she typed, Margaret noticed him in her window glass scribbling away in a loose-leaf binder. Curious, she peered forward and the young passenger moved slightly back. Perhaps he was writing in a journal. She often told her students to keep one. It put you in the mood, prepared you, like a prayer, for the work to come.
The train had turned away from the Hudson and was running now along the Mohawk, the pewter-coloured river which had slowed first to a trickle no bigger than a cow creek and then shrunk among the endless stands of maples and beeches to a string of puddles. The countryside looked just like Canada now, her True North strong and free, with its aerial stands of disputed softwood, and the scattery bricolage of its cities and sweet-mannered citizenry.

She turned away from the window and started over: "Darling boy, it did your old mother's heart good to see you and Tina looking so well. If you change your mind about New York, let me know and I will speak to Mathilde at UNICEF and see if they have work for you in Toronto . . ."

As soon as she typed Toronto, she knew it was wrong. Luke would take her offer of help badly; her son was, in some dreadful way, unhelpable. Tina said he'd lain stiff and uncomplaining while she dug out specks of glass from his back and scalp. It had taken days, Tina said, to get rid of it all.

Motherly concern would only make Luke's psoriasis worse. Margaret had seen the bumpy, sumac-coloured patches he tried to hide under his turtlenecks. Her poor baby. She felt badly for Tina too, but Luke was more vulnerable. He had always needed her more than her other children. And now he was lying awake nights imagining foreign invasions. His rants about the need for blockades on the Hudson and East rivers had shocked her. She'd hardly been able to restrain herself when he brought out his Manhattan subway map crosshatched with inky arrows indicating the bridges to use if you were planning to leave the city on foot.

Outside the window, stations were slipping by quickly. Albany was the last river town -- a fortress of high, chunky office buildings rising like penitentiary walls out of a wide, watery gorge. Margaret saw racks of windows glinting in the low rays of December sunlight. Then Syracuse, and finally, Rochester--the first lakeside city although she couldn't see a shoreline from her window. And neither, she guessed, could the young passenger behind her.

And then, at last, they were passing Buffalo. Shuffle off to Buffalo. When she was a child, her parents' generation sang this tune motoring south to escape the Canadian liquor laws. Now the people of Buffalo headed north for relief from their ugly little border town.

Anytime soon they would be going across the huge trestle at the border whose steel pilings extended several hundred feet into the Niagara Gorge. The attendant and her charge were rushing around, stacking up the uneaten cases of Doritos and soda bottles. The panicky trainee came over to Margaret's seat and opened the bin above her, hunting for lost potato chips.

The train stopped, and two American officials came on board. They looked like puffed-up adolescents with those eagles on their chests. But they were quicker than Margaret expected, and before she knew what was happening, the train had begun to inch across the bridge. She looked out the window and so did the young passenger behind her. Their eyes met in the glass; then his eyes darted off while she stared hypnotized into the chasm below. Down there, oh, so far down, the mighty, icy waters glittering a muddy emerald colour, whirling so fast, the rapids rising like soap suds above the green surface.

For a moment, Margaret imagined it had happened, and she was falling -- falling through wintry air, bleeding from the cuts on her leg and face. At this height, the river would feel like concrete. She would try a somersault if she could, like the tumbles executed by barefoot water skiers she'd seen skimming across the ocean's surface. But she wasn't as quick as these nimble champions and she was falling fast.

It was simple, falling down there. It was as if she had practised for this all her life, in her dreams, in her nightmares, standing on the roof terraces of tall buildings or peering over high cliffs by the lake.

Meanwhile, he was tying up the conductor and the Amtrak attendant. In her mind's eye, she could see him threatening to slit their throats.

Margaret was still falling.

Abruptly, the train stopped. They had come across the bridge. The Americans had been gentle with their questions, but the Canadian customs man in his pale grey overcoat sounded uncharacteristically rude. There was no reason for bad manners, she thought.

You! The customs official pointed at an older, dark-skinned man reading a newspaper. Where's your suitcase?

The Canadian customs man has another thing coming if he speaks rudely to her. Now the official was at the seat behind her, speaking to the young terrorist in his navy-blue windbreaker. Did she say terrorist? She didn't mean that. Sleeper. That's the term the news media used to describe people like him. He was so quiet he could have been a Canadian like herself. Everything about his person looked sucked in, as if he was pulling himself inward by his navel.

Have you been in trouble with the American government before? Where are your papers, the Canadian official shouted.

The young passenger whispered something in a gentle, plaintive voice. In a moment, he was on his feet. The official grabbed the young man by the elbow and steered him into the café car.

She hadn't realized he was Middle Eastern. Why hadn't she noticed this before, Margaret thought as he placed his knapsack on the table by the snack bar. He didn't look frightened. He looked confused, as if this wasn't meant to happen. She saw him raise his hands several times and gesture, pleading. The customs official was too busy writing to notice. They were joined by a second burly man in a grey jacket. This man seized the young Arab by the collar and shouted into his face. No one in her car could hear what the customs men were saying. No one moved, they were all watching, their faces registering shock.

She saw the handcuffs. Or did she imagine that? Certainly, he had his hands behind his back. Then he turned and seemed to give her a forlorn, knowing smile, as if she was forsaking him. She glanced away, her breath ragged. When she dared to look again, he was gone, and the train began to slide forward again, getting up speed so slowly that at first Margaret wasn't sure if it was moving. She was heading north, she was going home.

Susan Swan's Web site opens this week: http://www.susanswanonline.com. Her new novel, What Casanova Told Me, will be published next year. She is a humanities professor at York University in Toronto.