At the end of Mission 7: Transit Yards, after defusing all the bombs, you will be in a train with some enemies and civilians. Once you reach the end of the train, two enemies will take hostages and use them as human shields. If you get to close or start shooting, they will kill the hostages. To save them, throw sleeping gas. You can buy sleeping gas on your airplane (headquarters) at any time. Alternatively, you can also mark the enemies and use your execution ability to kill them immediately.