Honestly, if a terrorist really wanted to screw us, they could do it easily. The only thing is, it does not create fear, so they won't do it. with the 100s of substaions in a major city, all of which are not guarded, with in a couple hours 1 person, a car and off the shelf items could take out the grid for half a city with easy and the only way they would be cuaght is if we got lucky.