Boxcars, also known as Midnight, is the outcome of rolling the dice in a game of craps and getting a 6 on each die. The pair of 6 pips resembles a pair of boxcars 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 as for rolling any other specific ordered pair with the dice.
Boxcars (slang) Wikipedia
Rolling two 6-sided dice 25 times gives a probability of 0.505532 that at least once, Boxcars will appear.
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.