Terrain Cards are 45 cards that double routes. They were introduced in the Heart of Africa, but can be used for every map (though beware unbalances)
The 45 cards are divided into 15 desert cards (for warm colors), 15 jungle cards (for cool colors), and 15 mountain cards (for gray-scale colors).
At the start of the game, each player gets one terrain card. Then place the stack of terrain cards next to the train car deck. Flip 2 face up terrain cards. Terrain cards are picked like train cards and may be mixed and matched. All terrain cards are placed in piles in their terrain type in front of a player that makes it clear how many of each there are.
When claiming a route, a player may choose to use one terrain card to double the points earned from that route. But to do that he must have at least the same number of cards of said type as everyone else. 2 cards must be used to double a 4-6 length route, and 4 must be used for 7-9 routes. Any thing above that may not be doubled.