As far as I understand it, Tony Scott's Unstoppable has two characters, Chris Pine and Denzel Washington, who are trying to stop a train. The bad guys don't really seem to be a factor, nor do women, so far be it from me to explain how Rosario Dawson will fit into the film.
Production Weekly is reporting that Dawson is joining the production, which has started filming in the Pittsburgh area. My best guess is that she's playing Chris Pine's wife-- my friend working on the shoot mentioned a wife character popping up early in the film, though the role appears to be pretty minor. Has it been beefed up to accommodate a star of Dawson's caliber? Or following Seven Pounds, is she just taking the roles available until something better comes along?