Brad Pitt is "Ladybug" (all of our characters have names like this), an assassin who's been seeing a therapist as his poor luck has caused multiple unintentional deaths. As he's just returning to work, he's been given what he's told is a simple job to grab a briefcase and get off the bullet train at the next stop. Nothing goes as planned, with multiple assassins on the train all working at odds with each other.
As bizarre as this sounds, I'd recommend that if you're not a parent who already has a basic understanding of the main characters in Thomas the Tank Engine, you get yourself such an education before watching the movie. Although "Lemon" (one of our main characters) does go on at considerable length in the movie describing their characteristics himself.
A couple of us watching the movie made a strong connection to the work of Guy Ritchie (exemplified by "Lock, Stock, and Two Smoking Barrels") - the surrealistically comedic, clever and fast-paced dialogue, as well as the heavy involvement of luck in most outcomes. Unlike Ritchie though (I've never been much of a fan), I found this movie massively entertaining. Largely because the characters in this movie - despite being hitmen - are often fairly likeable.