@kishinfoulux: It's because by using the train, they accidentally attracted a herd, which is why you saw that huge crowd of zombies in the window at the end, and why the cancer group ran for the hills. I'm not sure why Vernon didn't tell them, maybe he thought since they were leaving soon anyway it would be alright. Seems like kind of a big plot hole. Of course this is only just speculation, so, take it with a grain of salt.

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