18?? is an 1870 variant with a new map, a large selection of private companies, and some major rule changes designed to make each game play differently. The base 1870 game rules are used. The game kit includes a summary of rules differences from 1870. The private companies vary game-to-game and players choose their company's starting city when a company opens. A copy of 1870 is needed to play 18?? because the kits only include the new parts.