The Ghau Amulet Thogchag is the fifth treasure that can be found in Uncharted 2: Among Thieves.
The Ghau Amulet Thogchag is found in Chapter 1 - A Rock and a Hard Place. As soon as you hoist yourself up into this train car following the previous flashback, don't go to the left (forward) to proceed with the chapter. Not before you turn back around and hoist yourself up to the far side of the train car, where you'll clearly see a shimmering object waiting for you to acquire it. Claim the Ghau Amulet Thogchag here, and then turn back around and continue on your way.