You must choose a map that it already has low rail length. I used the map with the island (i think bottom right at the map selection screen). Then you have to play the market as described at xbox360achievements.org until you have 7 billions. Go to bank and take some 1 billion loans and when you reach 10 billions the achievement will unlock. You don't have to play the game, just learn to buy and sell on market.
PS I do not describe the method for winning in the market with details as it is not mine solution and i do not get credit for it, but the basic rules are that you buy stakes at low prices (under 7 or 6) an d you sell them when the price is high enough (above 12-13). Also because the game sims everything you have to set the time to run as fast as possible. Hope it helps