hand holding the gun — and nobody notices because they’re too busy staring down at their smart phones and tablet computers. We’re talking about a train crowded with commuters and this guy is waving a gun around but nobody sees him, so engrossed are they in texting, tweeting and playing Angry Birds.
The author is referencing popular pop culture ideas such as tweeting and a famous game, Angry BIrds, and setting his claim that people aren't vigilant of whats happening around.