The chemicals at the center of the debate are called toxic inhalants (TIH). The most common are chlorine gas, which is used to purify water, and anhydrous ammonia, which is a key component in many fertilizers. Shipments of these chemicals represent less than 1 percent of the rail cargo moved through the country. Still, that amounts to an estimated 100,000 tank cars going through major cities each year. Homeland Security Secretary Michael Chertoff has called the transport of these chemicals "one of the most serious risks facing America's highest threat areas."
In 2006, the Department of Homeland Security implemented regulations that require trains carrying TIH chemicals to keep moving so they don't become easy, stationary targets. When they are stopped, they're required to be in a secure location. Their schedules are also intentionally kept random and secret. DHS is also working with the railroads on a system to track and identify the location of any one of these cars within three minutes.
Critics say these regulations still leave millions of Americans unnecessarily vulnerable to a deadly attack. That's because they do not mandate that railroads reroute TIH shipments around major urban areas.