"Derailed" takes the A train all the way to the end of the line. It's fast, slick, stupid, violent fun and, despite the cynically high body count, without serious intention in this world.

Derived from a thriller by James Siegel, it's based on this weighty truth: If it seems too good to be true, get the hell out of Dodge, Jack.

Say you're an ad man, whose career has just suffered a dispiriting bump. Say you're crushed by financial burdens, because your beloved daughter is a diabetic and the medical expenses are eating you alive, and they will never, ever go away. Say your wife is also a career gal, and she feels the same pressure from the finances, and the bloom -- sexual, spiritual, collegial, casual and even fun -- has gone off the union. You're tired all the time, you're depressed, it's gray midwinter, sunless, chilly, and that endless Chicago snow is turning yellow and black but will not go away as you rattle in and out, day by day, week by week, year by year, from Wilmette to the big town.

One day, you miss your commuter train. On a later train you're sitting there in your husk of misery, wishing life would be better, and suddenly you see a pair of legs in black stockings. Hmmm, you like legs in black stockings, particularly when they're thin and shapely and they cascade toward a pair of stiletto heels. Hello, you think, what's this? Then she smiles at you.

It helps to have some empathy for this forlorn macho ruin; if not the movie will make very little sense. But for us bent-down career boys with long commutes, it's a familiar dream. It never happens but we dream of it still, yearn for it, ache for it, sometimes see it in the last second before sleep, and when it does happen to Charles Schine (Clive Owen, hiding behind a passable American accent), everybody knows what he should do: Get off the frickin' train, go home, take a very cold shower, drink himself into a night of forgetful numbness and then get up early the next morning and catch the right train.

Charlie smiles back and there's your movie right there: It's David Lean's classic of middle-aged bourgeois romance, "Brief Encounter," with guns.

The guns come into play after Charlie has begun his affair with Lucinda Harris (Jennifer Aniston), who turns out to be an adviser at a large crosstown financial corporation. It's a tony place, she's a smart, chic woman, with an MA from Stanford and, like Charlie, she's somewhat worn down by life, oppressed by circumstances, and has a mate who no longer lights her fires and she no longer lights his.

So they're trysting the night away, having set up an elaborate ruse to fool their spouses, when suddenly the world goes out of whack: Feral man enters, stage right, through an unchained no-tell hotel door, in the form of Philippe Laroche (Vincent Cassel, who does feral beautifully), with an automatic pistol and a sense of cruel, Hobbesian domination. He pistol-whips Charlie, he rapes Lucinda, he takes money, credit cards, cell phones, dignity and any sense of the universe as rational. Charlie is ashamed that he could do nothing to overcome his adversary, yet he's now caught up in a mesh of lies; Lucinda insists that he not go to the police. Then Philippe calls him: He wants money. What can Charlie do? He is trapped.

Another question: What can filmgoers do? Every single one of them will suspect that some sort of con is being enacted, but the surprise the movie is rocketing toward is not that there is a surprise but what kind of surprise it will be. Is Lucinda in on it? Is it just Philippe? Is it a work rival, a wife who'd already turned against him? That's the game the director, Swede Mikael Hafstrom, plays off Stuart Beattie's script, as the movie waltzes toward the absurd, the unbelievable, daring you to break faith, then veering back toward some sort of just-marginal believability.

Looking at the 136 reviews of the book on Amazon.com, I see a fair reflection of the efficacy of the game. The novel gets mostly four- and five-star reviews, but every fifth or sixth appraiser gives it one or two stars and makes the same objection: Unbelievable. Strains credulity. Absurd. Come on, ridiculous. So your connection to the movie is likely to be determined by your ability to suspend disbelief. Most people can; about one in five simply can't.

The film overuses a couple of devices: The first is Charles Schine's uncanny ability to not merely survive but also participate in explosions of violence and then put some kind of instant spin on the events, convincing the almost immediately arriving police that he's not guilty of anything except being in the wrong place at the wrong time. Yeah, but . . . three times?

Then there's Charlie's quick mastery of his own violence. It takes the Marines 24 weeks and rigorous training to get good American boys to pull the trigger. Charlie learns overnight, and he gets really good fast. Too fast.

But the movie, at the same time, is unified by an idea. It's a reactionary conceit, possibly out of place in any society save that created by a thriller writer's hyper-fervid imagination, but always dramatically satisfactory. It's what might be called the First Law of Sociopathic Thermodynamics, universally applicable. It holds that if you're up against a sociopath, the only way to win is to become a better sociopath. You can't reason, you can't ask for mercy, you can't talk it over and reach an understanding. He will destroy you; that's what he does. Destroy him first or you die.

Charlie learns this lesson. As Jacobean revenge theater, then, "Derailed" is on its best footing. It documents how the tired, dispirited, joyless suburban dad finds his inner psycho and takes no prisoners.

Derailed (120 minutes, at area theaters) is rated R for extreme violence and sexual scenes.

A commuter-train flirtation between Charles (Clive Owen) and Lucinda (Jennifer Aniston) turns into an adulterous affair that leads to blackmail and then gunplay.