On the train there's a room you can climb to with a ladder which has a couple turrets, I think I'm supposed to use them to take down the helicopter. There were 4 soldiers inside, I sniped 3 of them from another room, but the 4th one is crouching down and is, apparently, immortal. I've emptied over 100 rounds in him at point blank range and he just winces but keeps living.

Edit: nevermind, seems I didn't trigger the story flags necessary to make him mortal >_>