Terry Harper wrote:

Seattle's system does this, running as trolley buses underground
through the city centre, and using diesel once on the surface. They
are articulated but I don't know who made them.

The Seattle dual power buses were made by Breda of Italy, but they are being
phased out due to maintenance and parts availability issues. Apparently, there
were only just over 200 made and they shared a similar heritage to Fiat cars
("Fix It Again, Tony").

The Seattle bus tunnel is currently closed for renovation for 2 years, and the
metro bus authority is taking the opportunity to replace the Bredas with deisel
electric hybird buses that cost $200,000 more than standard deisels and get less
than 4 miles per gallon.