Rising sea levels have caused floods on the New York City subway, killing thousands.
Another attacker exploded a bomb nearby, and a third man detonated explosives on a subway, killing 17.
Russia began discussing a color-coded system months ago, after two suicide bombers exploded in Moscow's subway, killing 40 people.
The most recent mass assault in Japan occurred in 1995, when the Aum Shinrikyo doomsday cult released sarin gas in the Tokyo subway, killing 12 people.
That was in March 1995, when members of the Aum Shinrikyo (Supreme Truth) sect preaching a bizarre distortion of Hindu, Buddhist and Judaeo-Christian beliefs unleashed a nerve gas called sarin in the Tokyo subway, killing 12 people and injuring several thousand.
In March, two female suicide bombers detonated themselves on the city's subway, killing more than 40 people — an act that the Chechen militant leader Doku Umarov claimed to have ordered, promising Russians that "the war will come to your streets".
Before Tuesday morning's mass killing, the deadliest attack committed on Japanese soil in recent decades occurred in 1995, when members of the Aum Supreme Truth cult released sarin gas on the Tokyo subway, killing 12 people and injuring thousands of others.
In 1995, terrorists had released sarin nerve gas inside the Tokyo subway, killing 12 people and sickening thousands of others.
The second incident occurred in 2005 after terrorists bombed the London subway, killing 52 people and injuring 700.
