Excerpts from a report in The Times on the Leeds proposals:

Trolleybuses are to return after an absence of 35 years in an attempt to rid cities of diesel fumes and tempt drivers out of their cars with the offer of smoother, faster journeys.

... modern articulated trolleybuses, which operate in dozens of European cities such as Lyons, Milan, Rome and Athens, have either batteries or small diesel engines that enable them to operate under their own power if the lines come down or the route is blocked. In normal service, they are much quieter than diesel buses and produce no harmful emissions.