Few monsters are more in need of a good lie-down than the zombie, a once-terrifyingly loaded cultural metaphor exhausted by repetition. Sometimes, though, you get lucky. The Korean horror film Train to Busan may not wholly reinvent the genre, but it does what it does with such exuberant, head-down momentum that it's tough not to get swept up and grossed out.
Using South Korea’s recent MERS outbreak as a launchpad, the plot follows a selfish fund manager reluctantly accompanying his precocious daughter on a birthday train ride. The other passengers soon become bitey. Writer/director Yeon Sang-ho smartly takes it slow at first, efficiently sketching the geography and supporting characters—expectant mother, love-struck high-school baseball player, a railroad executive who may as well be named Mr. Evil—while dropping brief shivery hints at things going awry in the background. Once the pace cranks up, however, something close to perpetual motion is achieved.
Yeon, an animator making his live-action debut, brings a disturbing, dislocated physicality to his swarming hordes, while also coming up with a rather astonishing variety of ways to cram them into seemingly every inch of the train’s increasingly slippery compartments. Even during those rare moments when the forward movement pauses (such as with a terrifically staged gag on a slowly descending station escalator), the utterly relentless, doomy vibe persists.
Such infernal energy generally comes at a cost somewhere, and, in hindsight, Yeon’s script does set up several provocative ideas about class and family, which it then disappointingly neglects to do much of anything with. (If you’re looking for that, there’s always Snowpiercer.) Any narrative deficiencies, however, ultimately pale before the sheer infectious rush on display. For a solid two hours, Train to Busan consistently brings on the half-giggly, half-appalling jolts, in ways that only a good horror movie can generate. Whatever your current stance on brain-eating, this is a keeper. 
