On the train stage after you climb up from under the train you usually have to go around the outside of the train by way of the side door. But that can be very difficult since if people in the train see you the mission is over. An easier way is to turn off the lights by using the switch outside of the room where you climbed into the train from. Once th elights are off one passenger will come to investigate but if you hide behind the last row of seats you can knock him out easily. After that you can just walk through the car past the terrorists and all. (If you want to once you get to the other end of car you can lure the terrorists to you by whistling and then knocking them out. Just make sure they don't trip an alarm.