The name of the bonus in the game is actually Longest Continuous Path. There are some DLC maps where the Longest Continuous Path bonus is changed to a different name or is not available, and those maps do not count toward this achievement. The bonus is awarded to the player with the longest continuous set of railways. While you can in theory get a length of 45, as long as your partner doesn't play any railways you should get this every game.
This can be unlocked in local play or online. The achievement will unlock after the final scores are displayed after your fifth qualifying game.