There’s a train. It’s long, with lots of cars loaded with toxic chemicals. And nobody’s driving it as it hurtles toward heavily populated parts of Pennsylvania.

And Denzel and Chris Pine are the only two guys who can chase it down and stop it.

That’s all there is to Tony Scott’s lightning-fast runaway train thriller, “Unstoppable” — “a missile the size of the Chrysler Building” and a couple of movie stars playing working class Joes who want to save the day.

And that’s enough. With Scott shooting and cutting this minimalist thriller to an hour and 35 minutes of heavy metal flying at you and the cool, collected old timer, Frank (Denzel Washington) and his distracted trainee Will (Chris Pine of “Star Trek”) jumping from one car to another, dodging hazards at rail crossings, this race against the clock works. The director of the limp “Pelham 123” remake takes a second shot at his railway movie, and this time gets it right.

Frank has decades of experience, so today’s shuffle of cars here and there along a short stretch of rail shouldn’t be a big deal. But elsewhere, others with less of a professional bent have made a boo-boo. A train “got away” from them.

“It got away from you?” the boss (Rosario Dawson) cracks. “It’s a train, not a chipmunk.”

As the “coaster” picks up speed, a welder is sent dashing off, by truck, to track it. Alarms are sounded. The BIG boss (Kevin Dunn) gets grumpy and the observing federal inspector (Kevin Corrigan) drops little pearls of wisdom about the physics of trains to Dawson’s hard-pressed line supervisor. A train load of school kids on an excursion is out there. Towns and then cities are on the tracks ahead of the “coaster.”

And it’s no longer coasting.

The film also features Scarborough native Ryan Ahern, an Army veteran who served two tours of duty in Iraq, in his first film role. Ahern, 25, plays a young Marine vet who works in the train yard and helps Washington try to stop the runaway train. It’s a fairly small role, yet he’s in on the crucial action. Because of his experience in the Army with helicopters, Ahern is seen in the film dangling on a rope attached to a helicopter hovering some 300 feet above the ground.

Scott’s film touches on the state of the American workplace (understaffed) and the American workforce (overworked, facing layoffs). But this is a straight-ahead ticking clock thriller, with the usual Tony S. trademarks — punchy dialogue and men doing what needs to be done. Oh yeah, and Frank is a proud father of two Hooters Girls working their way through college, Will has “issues” at home. Another Tony Scott touch.

It’s not as breakneck as it might have been. There are plenty of conventional pauses in the action while Frank and Will talk about their lives, their problems. Scott gets a bit carried away with identifying every single location, overly concerned with the geography of this “inspired by true events” story.

Washington and Pine have an easy rapport that makes even the soap opera elements go by easily. Mark Bomback’s script (he wrote “Live Free or Die Hard,” and the “Escape to Witch Mountain” remake) has an easy way with the jargon and a feel for the work — “In training, they give you an F. Out here in the real world, you get killed.”

And thanks to his previous subway movie experience, Scott never lets this express go off the rails.