Rachel takes the same commuter train every morning. Every day she rattles down the track, flashes past a stretch of cozy suburban homes, and stops at the signal that allows her to daily watch the same couple breakfasting on their deck. She's even started to feel like she knows them. "Jess and Jason," she calls them. Their life -- as she sees it -- is perfect. Not unlike the life she recently lost. And then she sees something shocking. It's only a...