The Tibetan Saddle Ring is the forty-eighth treasure that can be found in Uncharted 2: Among Thieves.
The Tibetan Saddle Ring is found in Chapter 14 - Tunnel Vision. After you breaking the next train car open. To continue, you will need to use the crates inside this train car to hoist yourself up through the ceiling. However, before doing this you can grab another treasure. More specifically, you can find this treasure at the far end of the locked train car. Once you've breached the train car, it's as simple as walking up to the treasure and grabbing it. Just try to grab it before poking your head out of the ceiling, which will catalyze a necessary gunfight, but one that might otherwise distract you from grabbing this treasure in the first place.