On Thu, 22 Dec 2005, Richard J. wrote:

But at least they could divert, which trams and trolley buses would not
be able to do.

Take a look at (apologies for the horrible URLs) some of the fleet
listed at
http://www.tpg.ch/Internet+TPG/Franc...cVehicules.htm
for example
http://www.tpg.ch/Internet%20TPG/Fra...HESSBBC-SE.htm

GMA (Groupe de marche autonome)
Moteur essence VW / 127

I read that as something like
"autonomous propulsion group / petrol motor", no?

I don't know whether this feature is ever used for rescues in
passenger service, but evidently these trolleybuses are capable of
moving themselves if/when the need arises. (Not all of them are shown
as fitted with this feature, in case you want to have a whine.)

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.