"Alan J. Flavell" wrote:

To the best of my recollection, some German trolleybuses have (or have
had) a fully fledged diesel motor, used routinely on the outer parts
of their routes, and only switch to/from OHL power for the more
central parts of the town/city.

There's an interesting video here

mms://winmedia.metrokc.gov/transit/ITtunnel.wmv

It shows dual operating buses and some other detaiils.