Boxcars, Midnight & a Freight Train

Boxcars (also: "Midnight") is the outcome of rolling the dice in a game of craps and getting a 6 on both dice. The pair of 6 pips resembles a pair of boxcars on a freight train.

In some role-playing games, 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

from http://en.wikipedia.org/wiki/Boxcars_(slang)