"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. Evidently, in the event of a problem
(road blockage, OHL or power failure) they would be capable of
continuing in service on the other power source.

As do some in the US - Seattle among other cities use them. Interestingly
enough, it appears that they are being phased out though. Note the last
paragraph:

Planned rebuilding of articulated trolley buses: The dual-powered
(electric-diesel) articulated buses that go through Seattle's bus tunnel are
reaching the end of their useful lives. Metro plans to rebuild them as
electric-only articulated trolley buses and bring them into service in 2004-2005
on high-ridership routes 7, 43, and 44.

http://transit.metrokc.gov/up/archiv...4-trolley.html