A simulation game where the size of the game is the continental US and you have to lay track to connect the east and west cost.
Unlike other train layout games where the size of the game is only a few mile square this would be the entire continental U.S. so planning where to cross rivers, which states to go through and how to work around the mountains matter. Starting out small connecting cities to earn money, expanding east and west connecting cities, and creating cities just by creating a stop until you have a continental train track. Mountains, rivers, bridges, tunnels, grading, route planning, engine/car purchasing, wood/iron acquisition, stock trading and everything about 1800's train route planning mixed in.
And of course disasters! From train wrecks, mountain slides, snow, washed out bridges, train robberies, and more.
Given the size/scale of the U.S. there could easily be some A.I. competitive modes where you are playing against other train companies and even potentially buying/selling/merging with them, not to mention racing to connect the two sides of the U.S. along with other big cities.
The game mechanic could be be several different things, but at lease one interesting bit would be the surveying / route planning. Given an interface similar to Google earth you would be able to zoom around looking for cheaper and faster routes.