From http://riograndegames.com/games.html?id=189

The postal carriages continue to roll. Now that players have learned to master the postal routes in the south, they naturally turn their eyes toward the north, looking for more routes to establish to add to their ever-growing postal networks. The new routes run between Holland and Sachsen - between Preußen and the free cities. The players build new postal stations in order to provide fast service for important letters to the many new customers in the north.
Hard-working postal carriers add horses to their carriages to enable them to travel farther and more safely, which will help the separated Preußen provinces to achieve power and glory.

With this expansion, we offer new ways for players to enjoy Thurn and Taxis. The new board opens up new areas for postal routes. To support the new board, we include new bonus tiles and new city cards (with horses on their backs to support the new rules). We hope players will enjoy these changes and the strategic opportunities they offer.