A bet that pays off big or just something that is awesome. Comes from the racetrack, where a winning bet on a big ticket pays so many zeros that it looks like the dots on double sixes ("boxcars" in dice). Can be used in combination with ship it.
Man, I hit that moneyline parlay and it paid boxcars.
Dude, did you see that girl's ass? It was boxcars.
Guy 1: We just scored an 8th and it's nuggy.
Guy 2: Boxcars.
Guy 1: That slampig wants to f you later
Guy 2: Boxcars, ship it.