Standard
Marked owner may play elsewhere
You may still play on the Mexican Train or another player's marked personal train.
Marked personal trains
A marker opens your personal train to other players. If another player plays a domino on it, the marker stays in place. It is removed only when you play a domino on your own train.
Stricter Option
Marked owner must play their own train first
While your train is marked, you must play on it before you can play on the Mexican Train or another player's marked personal train. If you cannot, draw one domino from the boneyard. Play it if you can; otherwise, pass.