Kill all Watchmen before they reach the Railcar. [Bronze / 20G]

Location: Regina
During the "Regina" chapter you will be driving a railcar. At some point you will have to push another railcar out of the way. That's when the Watchmen (mutated dog-like creatures) attack you. None of them may reach your own railcar, so you have to kill them very quickly. You might want to use your military rounds instead of the normal ammo. It does a lot more damage and will make this a lot easier.