There is a platform/hallway that goes over the center of the map. To get to it, go behind the trashed locomotives. Climb up the staircase and turn left at the round platform. You should have a decent view of the whole roundhouse in the middle of the map.
There is a crashed locomotive by the ladder that leads to the platform. Crouch inside and you can snipe without anyone noticing.
On the other side of the platform you'll find another crashed locomotive. Do the same thing as above.
Go up the stairs that leads to the platform. At the top, instead of turning left, turn right to see a hole. Crouch and go through the hole. You should be on a train. From here you can kill people going up the stairs to the platform.