[nerd_light]

Ah, I think we're both right here, just on slightly different things. You're correct, trolley with diesel backup isn't new and is in fairly wide usage around the world. To the original comment:

"retaining the ability to drive disconnected from the wires and constantly charge when in an area with the wires"

Seemed to refer to trolley buses with rechargeable battery backups, which are much less common. Admittedly, I don't have non-US stats, just that there are only a handful of cities in the US that use them.