when Soth finish his conversation, where do i have to go for the extraction and how?,

Thanks jzorro

You just have to sneak past the guard that's left and head for the next section ahead (past where Soth made the phone call). An alert may sound and you've got a few seconds to head around a corner to the right and climb a ladder to get on top of the train.

Start running for your life as a chopper will start firing on you. Once you reach the Osprey there will be a rope for you to jump to/grab which will end the mission.