First what is a train when it comes to playing dominos?
The object of the Domino game is for a player to play all their dominoes from his or her hand onto one or more chains, or "trains", that branch from a central hub or "station".
The Mexican Train is an additional train that anyone can play on when it is their turn.
They can start the train by playing a domino matching the engine (i.e. the double played at the beginning of the round) or add to the train.
Mexican trains can be added as long as there is space that remains.
The double 12 game set is often the standard for the Mexican train.