At the start of the level simply follow Clay to the destination, once you are there place the TNT barrel onto the target area. After the cut scene follow Clay to the train, and when you reach the train kill all of the enemies on and around the train, eventually more enemies will arrive on horseback. Again, quick draw is very helpful here. When you are done killing, back track all the way to the tunnel area, where you first started. (Make sure you get there before the train). Place the last TNT barrel, and wait at the top of the ledge, be ready to shoot the TNT barrel when you are told. Once the train crashes you will need to eliminate all of the remaining enemies on the train. Once everybody is dead the achievement will unlock.