Ask MTD: "If a player never starts their Mexican Train, can anyone else play on it?"

The question: Debate centers around whether the player without a personal train can claim their empty spot is "reserved", and thus can't be used for a train.

A Reddit thread sparked this question in Mexican Train gameplay:

It's nearing the end of a round, and one player has not yet established their personal train. Thus, their alloted train space remains open. Another player draws a domino that could start on that player's personal train. But if they've never started their train, does that mean they can or can't receive dominoes?

a Mexican Train gameboard showing a player train not started.

Our answer: The empty train's open spot (the spot after the engine) is not inherently reserved, so other players can play on it whenever it's open. A train is open as soon as a player can't lay a domino on either their train, someone else's train, or the public Mexican Train.

Any of the Trains can start at any time, and the position on a train, if open or playable.

a Mexican Train gameboard showing a player's train as not started but open for other players to play on. — In this opening move, Butch Cassidy lacked a playable Double-12 to start his train and a playable tile for any other train (which are mostly empty). Thus opening his train on the very first round. Player 4 has the ability to lay her single Double-12 on her train, the Mexican Train, or Butch Cassidy's.

Remember:

All trains always start with a double-12 domino
Personal trains are closed until a player can't play a domino on the table. Then they're open, whether they've started their train or not.
If a person hasn't started their train, their train becomes open, and someone elses can lay a Double-12 to start their engine, that's a fair play.

a Mexican Train gameboard showing a player playing a domino on an open train. — Here, Jesse James plays a 6|5 domino after Butch Cassidy laid a 12|6 on the single, open train after the fourth player couldn't play on any available train. This is a relatively poor strategy for player 4, since they opted to play mostly on the public Mexican Train, and any chance of stringing playable tiles together was lost as other players joined in.

This is an interesting question, since it's rare to get far into a game of Mexican Train Dominoes without starting your own train. Presumably, that player has been playing exclusively on the public Mexican Train and on other players' trains. Or they've been drawing a lot of dominoes! And it's not a great strategy, since most players want to line up their dominoes for maximum doubles to win another turn and play more tiles. It also reduces your playable options by 20% later in the game.

a Mexican Train gameboard showing a player not starting her train but instead playing on the mexican train. — In this round, Player 4 has chosen not to start her Mexican Train, and, after laying a Double-6 on the Mexican Train couldn't cover it. She drew a tile, Pancho Villa was forced to cover the exposed double, and Player 4's train was open briefly until she played another pass.

Furthermore, when playing a double or on your turn, everyone must take some kind of action. Even if you pass because of a lack of playable tiles, that's still an action being taken by drawing another domino, which also opens your train.

Our online Mexican Train Dominoes game follows these rules. It's an obscure scenario and question that isn't explicitly written into any typical MTD Rules Document, but several rules are clear and still apply:

Every player takes some kind of action on every turn, such as...
...passing a turn results in the action of drawing a domino
Not being able to play on your own train means your train is open
Nothing stops other players from playing on any open train
If you have a playable tile, you cannot knowingly can't skip playing it in favor of drawing a domino (our online game won't let you draw or pass if you have a playable domino somewhere on the table)
If someone plays a double and they can't cover it (match it) on their same turn, their train is open and no one can play anywhere else regardless of any other train's status until the exposed double is covered.

Is a personal train automatically reserved for its owner until they start it?

No. A personal train is only protected so long as it's closed.

What if the player chooses not to start their train?

Doesn't matter! A train is playable when the train is open.

Can another player place the first domino on someone else's train?

Yes, so long as the domino matches the engine and the train is open.

Does this rule apply only to the end of a round?

No, it applies during any part of the game.