Boxcars is the outcome of rolling the dice

Dice

A die is a small throwable object with multiple resting positions, used for generating random numbers...

 in a game of craps

Craps

Craps is a dice game in which players place wagers on the outcome of the roll, or a series of rolls, of a pair of dice. Players may wager money against each other or a bank...

 and getting a 6 on both dice. The pair of 6 pips resembles a pair of boxcar

Boxcar

A boxcar is a railroad car that is enclosed and generally used to carry general freight. The boxcar, while not the simplest freight car design, is probably the most versatile, since it can carry most loads...

s on a freight train. In modern parlance, it refers to such a roll in any game involving 6 sided dice which are marked with pips. The probability of this roll is 1 in 36, or about 2.8%. This is the same probability for throwing any set of pairs with the dice.

Probability

Rolling two 6-sided dice 25 times gives a probability of 0.505532 that at least once, Boxcars will appear.

Betting

One of the 4 bets on any common craps table. It is on the 'high side' of the horn bet in which the player can either bet on the 2, 3, 11, or 12. When the player throws in money and calls out "midnight" or "Boxcars", the dealer will usually 'book' the bet as a bet on the 12, so as to not create confusion.

Related terminology

In some role-playing game

Role-playing game

A role-playing game is a game in which players assume the roles of characters in a fictional setting. Players take responsibility for acting out these roles within a narrative, either through literal acting, or through a process of structured decision-making or character development...

s, especially ones where a roll of 3 six-sided dice is a standard action-resolution mechanism, a roll of three 6s is often referred to as a freight train. In roll-low systems (such as GURPS

GURPS

The Generic Universal RolePlaying System, or GURPS, is a tabletop role-playing game system designed to allow for play in any game setting...

), where a roll of 18 represents a critical failure, or in In Nomine

In Nomine (role-playing game)

In Nomine is a role-playing game designed by Derek Pearcy and published in 1997 by Steve Jackson Games, based on the French game In Nomine Satanis/Magna Veritas. Players typically assume the role of angels and demons in a setting that draws heavily on the traditional Christian mythos...

, where a roll of 6 6 6 represents infernal influence on events, it is referred to as a "Freight Train from Hell."