When do I mark my train?

If you cannot play a domino from your hand, draw one domino from the boneyard. If you can play that domino, play it. If you cannot play the domino you draw, mark your train if it is not already marked and pass. Similarly, after playing a double, draw one domino from the boneyard if you cannot play another domino. If you cannot play the domino you draw, mark your train (see Doubles).

Read the full rule: When you cannot play