The St. Petersburg Metro blast is my number one fear during my commute. The DC Metro is wide open to shit like this. Even when they do attempt to institute an illusion of security, it's spot checks at best and all you have to do is go a few miles to the next station.
Of course, there's no real way to better defend the system, I guess. To be fair. Still, though. I'll go weeks without seeing a transit cop. So even the simple deterrent of having a police presence isn't a thing.