Since backgammon is a game where you have to use dice, it is necessary to learn some specific words which relate to the dice. So, the term boxcars relate to dice rolling. Actually it is used when one of the players rolls double sixes. In some variations of backgammon or simply in some different areas, the terms boxcars is not used, but there is another specific word, Boys (or The Boys) or Boxes, but they also mean a roll of double sixes.