It’s war. There is heavy fighting between whateverland and its southern neighbour. To support the war, a constant flow of supplies and fresh soldiers to the front is necessary. Railroads are construct to facilitate the supply, but like towns and cities may be lost if insufficient supplies and soldiers are delivered.
In this 3-player variant of Age of Steam, the players are responsible for building railroads and making deliveries to the war front. Depending on their success to do so, the frontline moves, and therefore, the map changes. Because its war, they can all lose, but only one player can win.