Funny thing is, not alot of people I play can rape train, majority of players dunno how to play, normally go down by round 4. Rape train is essential to get to high rounds. Yeah, if you know how to do it, it's easy, but the thing with that is you can easily get lazy, not focus due to boredom and that's how a rape train can screw up. Looks like Mob of the Dead could fix that with all the narrow corridors (like Shangri La.)
They know people rape train and try to break the trains by putting in walkers at any round, as opposed to just having nothing but running zombies. If it was cheating or hacking or anything, they would of done more extreme methods to eliminate it, like changing the zombies AI so that they split from the pack to intercept you rather than just follow you.