To find the Putrid Subway Car Keys needed to open the Quarantined Belongings lockbox in Dead Island 2, travel to the Metro at the spot shown in the image below.
Once there, take down the big guy then loot the Putrid Subway Car Keys. Now that you have the keys, head to the nearby train car and head inside.
Loot the Quarantined Belongings lockbox inside this train car to receive the Puncturator Brass Knuckles.