The same problem is in other coutries. For example , in 2005 polish military magazine "Komandos" published article about possibe terroristic strike in Wroclaw [I study there]. In this scenario, RPG-7 would be fired to train carring chlorine. 180 tons of chlorine would cause death of about 40.000 people.
It is also possible that chemical plants can be attacked to achieve similar effect but, trains are easier targets.