Trolley buses are necessary on many Seattle routes because of all the hills, you just can’t beat torque on a good electric (same is true for many other hilly cities like SF, Vancouver, and Lausanne).

Metro and sound transit has also invested in a lot of hybrids. The most recent fleet is pretty interesting (they also have a few battery-only electrics, but these seem more like testing).

I work for https://www.proterra.com, King County has number of our buses in service.

> The tech exists, but isn't in wide usage.

The buses in Lausanne when I lived there were on overhead electric when in the city center and switched back to diesel when they came off the wires. The same was true in a number of other Swiss cities. So I would hardly call it "not in wide use." Just not in the USA.

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.