Thought I would give the suicide a try. This allows me to either (1) reload last save, or (2) reload last checkpoint. Option (2) would work for most folks if they suicide soon after the bear gets stuck. However, after the bear got stuck, I foolishly left the zone and worked on Jeremiah's letter burning quest. This caused the checkpoint to move forward in time, which means both my normal save and the checkpoint save both have the bear trapped in the rocks. So, I'm still stuck with no way to advance the story. Phooey.
I've also been stymied by this quest. The old bear seems to have fallen through a paper thin crack in some rocks near the derailment. Now he's just stuck there, running in place with only his back half sticking out of the rocks. Sadly, shooting it in the cheeks doesn't actually count for the 5th or 6th shot. Harrumph. So now I have a save game with the old bear stuck in the wall, and I cannot progress the story. Maybe I'll try wandering down to the Muskeg to burn those papers; with a hope that the bear will respawn when I come back to this zone.