The threat of terrorism in Europe has been on the rise in recent years, with experts and politicians particularly worried that terrorists might make use of dirty bombs. Fraunhofer researchers have developed a new system that will be able to detect possible carriers of radioactive substances, even in large crowds of people. This solution is one of many defensive measures being realized in the REHSTRAIN project, which is focused on security for TGV and ICE high-speed trains in France and Germany.