For residents of southeast Paris, the construction vehicles rumbling back and forth behind the Austerlitz train station are a loud annoyance that has gone on for too many months. But for city officials—and countless Parisians, they hope—history is unfolding behind the cordoned-off area. After years of thwarted ambitions and vague promises, the French capital, officials say, is set to accomplish a rare feat for a major metropolis: making its once heavily polluted waterway fit for swimming again.