No need to imagine. Sarin attacks were carried out by (rogue) scientists in the Tokyo subway in 1995:
The Sarin attack on the Tokyo subway, usually referred to in the Japanese media as the Subway Sarin Incident (地下鉄サリン事件 Chikatetsu Sarin Jiken?), was an act of domestic terrorism perpetrated by members of Aum Shinrikyo on March 20, 1995.In the confined space of the Tokyo subway, those attacks generated an average body count of 2.5 people per attack. In the open air environment of a marathon, I'd be surprised if the casualty count were higher. During WWI, troop casualties from chemical weapons rivaled those from high explosives because soldiers were entrenched, and heavier than air gases like most chemical weapons tended to find the lowest point in the ground, where it pooled along with the soldiers sheltering there from bullets and artillery shrapnel. Any attempt to exit the trenches was met by machine gun fire from the enemy.In five coordinated attacks, the perpetrators released sarin on several lines of the Tokyo Metro, killing thirteen people, severely injuring fifty and causing temporary vision problems for nearly a thousand others. The attack was directed against trains passing through Kasumigaseki and Nagatachō, home to the Japanese government. It is the most serious attack to occur in Japan since the end of World War II.
I had just departed Japan when it happened. One of the casualties was a train attendant, who saw a bag emitting a cloud of poison, grabbed it in his hands, wrapped it in newspaper and carried it to a locker. It cost him his life. His name was Kasumasa Takahashi.