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.
If you do not wish to purchase the game set but would like to play with standard dominoes, check out these Mexican Train rules and strategies by David Bauguess. You can also find Mexican Train Dominoes rules and game variations at Wikipedia.