Activists eluded thousands of police last night and halted a train carrying nuclear waste in northern Germany by cementing themselves to the tracks. The incident was one of many protests against the transport of the waste, which originated from German nuclear reactors and was sent to a French reprocessing center in the 1990s. Officials from the two countries suspended shipments back to Germany in 1998 because of concerns about leaks, but resumed them this week, promising to tighten safety precautions. Activists at other points along the train’s route last night fired flares and threw stones at police, who tried to disperse the crowds with water cannons.