It's about trains (which in and of itself probably appeals to a few of us), and the goal is to hook up train cars to a train as it's moving by tapping on hooks at exactly the right time. The gameplay's simple: you just tap on the hooks when they line up about every five seconds or so. The graphics and sounds really fill in the background, though. The trains are designed on real cars, they move through various beautiful backgrounds, and the sounds of click-clacking on the tracks and the trains hooking up really get you in the mood. OpenFeint integration and special bonus levels can up the ante as well.
There are a few graphical quirks. For example, they went with 2D models for the trains, so when they turn, it breaks the illusion a bit. Iron Horse is far more fun than it has any right to be, though, especially if you're a fan of trains. The full game is US $1.99, and there's a free version to try out on the iPad.