Unmissible Storyline achievement. Can be completed on any difficulty.
While outside the mine, wait for the loudspeaker to mask your shots. Make sure to take out the sniper next to you, and the one across the map. You might what to set some traps on the road. The entrance to the train area is on the right.
Once you're in the tunnels, it will be harder to hide. Save your traps until immediately before you enter the office. You will be ambushed there.
Once you've defeated the ambush, you need to return to the start. There will be some scattered enemies in the tunnels and then two truckfuls at the end.