Mini Metro is a simple game about trying to design an efficient train system in a rapidly evolving city. It's represented by a basic train map, and you change routes just by dragging your mouse between stops. There's a free alpha that you can play right now!
On the game's Greenlight page, there are plenty of caveats about necessary difficulty tweaks, bugs, and upcoming features, but the game is fun even in its current state. Play it for free in your browser here.