Stephen Richards schrieb:
but they sound just like a very quiet diesel engine, with very definite
gear changes. Can anyone explain what technology is used?

I might be able to explain the gear changes. I suppose these buses are
part of this special european fuel cell project. We have a few of them
here in Hamburg, too, and the bus company explained to me that it was a
standard diesel bus with only the diesel parts removed and an electric
motor dropped in, leaving the transmission and gearbox and everything
intact.
This was done to make the bus as compatible as possible to diesel buses
of their type, and, it was said the conversion was cheaper than a fully
electric bus (why they did not just take a trolleybus design is beyond me).

They said that when fuel cell buses would go into full use, the design
would be changed appropriately.

The current fuel cell buses even kept their standard brakes, dissipating
a lot of energy into heat when braking which could be turned back into
electricity instead of heat...