It might just be a nutcase. But consider terrorist modus operandi and goals
Terrorists frequently attack public transportation, because they want a population to grow fearful, stop their traveling and their routine, doing damage to the economy. They want people to be depressed and to doubt the ability of the government to protect them. They want a fearful public pressuring the government to cut deals and compromise with terrorists and their state sponsors, and to stop helping governments in opposition to the terrorists, in exchange for peace and security.