Some buses run on batteries, but I've seen several systems now for buses that get power from overhead lines (similar to trains). The summary seems to be overlooking these vehicles.
While overhead wires are fine for trains, which have predictable, smooth, well-maintained paths to travel, they are less than optimal for buses.
We have these in Cambridge, MA. They are a hassle because it's a fairly common occurrence that the armature will pop off the overhead wire and the bus will grind to a halt until the driver can go around and use a pole to hook it back on, creating a huge traffic hazard in the meantime. It would be great if they had some battery backup and could limp along to a bus stop before having to be reconnected.
Also, during power failures, every bus stops. This is great when two buses happen to be passing in opposite directions, causing the entire road to be blocked.