Zombies and Trains is the latest app from Dragonhead Games, an indie dev based in Norway. It's a zombie app (which is fairly meh these days), but it features a pretty inventive control scheme and a whole lot of content to play through.
The app is essentially an arena game, where you're presented with wave after wave of zombies, and then need to kill them off. Rather than using a gun or a hatchet, however, you use something a little more powerful: A freight train. Each area has various tracks running across it, and just touching one of those tracks will send a train rumbling through, demolishing any zombies who happen to be in the way. The game can be frantic but very satisfying -- most of the stages involve a trophy in the middle for you to protect, and if even one zombie reaches it, it's game over. There are four different modes, including a progressively difficult Arcade mode, an endless wave mode, a one minute survival mode, and a "slaughter" mode, which just has you killing as many zombies as possible before time runs out.
The whole thing is fairly simple, but the graphics are well done, and the idea of a train at full speed running through a herd of zombies is really just too good to pass up (especially if you've played The Walking Dead game, which combined zombies and locomotion in a much less satisfying manner). Zombies and Trains is available for 99 cents on the iPhone, or $1.99 on the iPad.