For the entire time, a certain group from the Catalysts is working behind the shadows, and they are being led by both Rupert and Harley.
The whole plan is to lure as many zombies as they can into the current location of the special elite force. By doing so, they can lead them further and further away from the armored train where they can resupply their weapons. It's a scheme that can risk many people's lives, but it's necessary for them to meet ends.
As for how exactly they were able to lure the zombies, they made use of their sharp sense of hearing. Way back before the mission commenced, they secretly replaced the soldiers' weapons with similar-looking guns that were louder than average. They're not the usual armories that they used.
