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In this episode of Border Wars, agents and officers struggle to stop cartels and illegal immigrants who've found weak spots in spite of increased fortification along the U.S.-Mexico Border. Four feet beneath Nogales, agents crawl through a claustrophobic drug smuggling tunnel, knowing they could come face to face with armed criminals. Near the Port of Entry, Nogales' Rail Inspection Team is on high alert as a freight train full of new automobiles rumbles toward them. Trains, with infinite hiding places, are becoming a favorite place for cartels to stash drugs: If the officers aren't careful, they could easily miss a load. In the desert, Border Patrol's only dirt bike unit races after a group of illegal immigrants. But in this sprawling Indian Reservation, the agents will need skill - and luck - to catch them.