The less you know about the film, the better. All I knew is that it involved an American couple who were traveling on Russia's Transsiberian railway.
Most mainstream thrillers are crap, formulaic and disappointing. But this one constantly surprised. I found myself much more tense watching it than I typically am when viewing a movie; very effective work by the director. If interested in the film, don't watch the trailer; it reveals way too much.