GTA: Zombie Train
The world has succumb to the zombie apocalypse. Luckily you are able to hide in the train, but the problem is that it is being attack by zombies. Armed with a hand gun, you fight for your life while the train moves. You need to defend yourself in the train as the zombies would budge in and would try to eat you. Make sure that none of them gets close to you. Each kill earns you money, which you can use to buy some upgrades for your character. The game would be harder as you make some progress, but it would also be more exciting than before.