To get the full gist of what happened, read these three articles since each highlights different things:
One would think, being Washington DC of all places (Homeland Security and all), WMATA would have a full array of security devices on its fleet. Not too much of a stretch when lots of problems have occurred, and some DC neighborhoods the buses serve are more than just rough, they're downright war-zones.
Even if this bus had been equipped, it doesn't sound like it would have discouraged the perp.
It'll be interesting to find out if this character has a lengthy rap sheet, and if they'll play up some mental problems or substance addiction to lighten his sentence.