“Casino Royale” director Martin Campbell is in negotiations to board “Unstoppable,” a 20th Century Fox drama about a runaway train carrying a cargo of toxic chemicals.Scripted by “Live Free or Die Hard” scribe Mark Bomback, the drama pits an engineer and his conductor in a race against time. They’re chasing the runaway train in a separate locomotive and need to bring it under control before it derails on a curve and causes a toxic spill that will decimate a town. Julie Yorn is producing for Firm Films, along with Mimi Rogers. Chris Ciaffa is exec producing. Fox has the project on a fast track. Campbell’s repped by CAA.