I read about this iOS game called Trainyard the other day and didn't think much of it. But then I read another post about the game and how fun and addictive it was, so I decided to try it out.
The premise is really simple, you've got to get trains from point A to point B. Along the way, you'll need to change the color of the train as well as change the number of trains that reach the station.
You accomplish this by laying down track and changing the order of the "switches" so that the trains go where you want them to go. There's no time limit, which is good because it's all a matter of trial and error.