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.

Which routes in Lausanne had that? I never noticed any dual use buses when I lived there. It isn’t surprising, Seattle used to have dual engine buses in the 80s/90s that were diesel until they went into the downtown bus tunnel. They got rid of that when they brought the train in, and I guess the buses were cleaner by then as well.

I lived there up to about eight years ago, and I recall most of the routes running through the center city were dual. I couldn't name route numbers at this remove, though.