Once in control of Artyom, head out of your room and follow Krest to the front of the train. Once there, interact with the chair to have a conversation with Miller and the crew.
Once that's done, head towards the back of the train. When you get to the workbench, immediately turn around and look on the edge in front of you (directly across from the workbench) for Diary 63/71 (Katya's Note). Head back to the map and choose to progress to the final mission.
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