The gist of Pandemic Express - Zombie Escape is, relatively speaking, the usual. A group of human players try to outrun a (human-controlled) swarm of infected, only everyone spawns in a train station until one person is randomly infected and begins turning fleeing survivors into zombies, who are trying to escape by train. Survivors must get to the marked train, looting the map for weapons and items along the way, and then escape on the aforementioned train. Zombies must stop the train by keeping the survivors away from it, killing them off and infecting everyone, which bolsters their ranks and makes killing the remaining survivors that much easier.
The game is obviously inspired by "Zombie Escape" type from mods in games like Counter-Strike, Halo, Call of Duty etc., ad nauseam. Basically, they want it to be what would happen if Left 4 Dead's swarms of bitey zombies were actually human players. It's not a bad premise, and it's been done in one form or another a bunch, so there's a template to follow and expand upon. The question is whether or not it cane be done better (or at least as good) here on Pandemic Express.
The game launched today on Steam, and you can watch the train insanity below with the launch trailer.