Runaway projectiles have always made for intriguing character studies — think Speed or Runaway Train. Unstoppable is that kind of movie, and it's based on true events to boot: how a choo-choo — owing to a combination of incompetence and bad luck — becomes an unmanned missile (a half-mile of fuel and flammable toxins) rampaging at 70 mph through the depressed burbs of Podunk, Pennsylvania. Early rescue attempts fail, so Johnny-on-the-spot engineers Frank Barnes (Denzel Washington) and Will Colson (Chris Pines, a/k/a the new Captain Kirk) jump into the mix but must battle their yin-yang differences as well as the steel beast. The film saddles the impromptu duo with carloads of backstory, but the actors are on their game — and that includes Rosario Dawson, who adds a deft touch as the track manager fighting corporate inanity. As for director Tony Scott, who's always been all over with his camera and cut-happy edits, he here finds a rhythm that delivers suspense from nose to caboose.