Kepler & K2 provide long-baseline, high-precision photometry for exoplanet and astrophysics research.
The basic and most common additional rule for Doubles play is that where a Double exists at the end of any train, the Double must then be 'satisfied' (laid against) in the next turn, even if the Double is on another player's train that is not marked.
This domino may normally be played either. 1  on the player's own train, or. 2  on another player's train, if that train is marked with a penny, to show that its owner was unable to play on it at their last move, or.  on an extra train, known as the Mexican Train or Caboose, which is always open to all players.