When a player places a double domino in Mexican Train, the player must place another domino down. However, there are several variations when playing dominos with the Mexican Train rules.
The rules for Mexican doubles are tricky and recommended for advanced players. The most common rule of Mexican Train doubles is that if the double has not been played on by the end of the first person's turn, the next player must complete the double even if their train is not on the double. If the player cannot play a domino, they must draw from the face down domino draw pile. Depending upon the variation, the player either draws one domino or draws until the player can lay down a domino. Sometimes this move could be considered unable to happen, but because of the double rule, the player still must lay down a domino to complete the double.
When the second domino is played, if it is another double, then the player must again play another domino. This can be lead to one player laying down several dominoes at a time, allowing them to run out of their dominoes and win the round. Along with this rule, some players complicate this rule even further and make the rule that if a player lays down two or more doubles, they must lay down the next domino to finish the line in the opposite order. The only time another domino does not have to be played is when the double being played finishes the line.