[grandalf]

It would be very easy for a single unskilled attacker to kill upwards of 20 people using a conventional firearm or stuff that can be found at any hardware store.

I'd estimate that such an attack would result in the subway being closed for weeks, particularly if several happened successfully a few days apart.

Similarly, small explosions dispersing any remotely harmful smoke or chemicals (even chemicals that are easy to obtain) would be amplified by media coverage, and politicians would close the subways as a knee-jerk response.

Sure, an actual banned bioweapon/chemical would be worse, but my point is that terrorism (the tactic not the alleged existential threat) works because it requires little skill, technology, or access to difficult-to-obtain materials to create fear.