In terms of the potential for economic damage, a full-fledged bioweapon attack is not all that much more effective than a low-sophistication attack (or several, staged at the same time). So while it is useful to understand the way subway systems may disperse particles, this kind of research does not reduce the risk of economic damage from low-sophistication attacks targeted at the subway system. It is difficult to ob…
Fear is a huge part of any terrorist endeavor and small invisible particles killing you while you commute is far scarier than someone with conventional arms. Further, NYC subways would be closed for weeks, the US's largest city and an economic powerhouse would grind to a halt. The economic damage is such an event would be huge.
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.