Assaults on mass transit in the US aren't what I'd call a normal state of affairs. I haven't seen it yet in person, though clearly it happens. Petty theft is a good bit more common, though like 99% of the time I just see people doing their thing and ignoring everyone else on the bus/train/whatever.

I read comments on HN and kind of wonder if this is why people believe all these terrible things about the US. Never been here, and only have comments online to judge by. Explains a lot.