Trolley buses are a weird niche. They require permanent overhead cable infrastructure like trams do but don't have the other benefits of trams - higher capacity, greater speed, better ride and no tyre pollution. I figure if you're going to install a trolley bus route you probably might as well install rails at the same time and get the benefits of trams. (Aka streetcars for the North Americans out there)

The City of London did assessments on trolley buses and found that the added capacity of trams made them the better choice pretty much anywhere trolley buses were proposed, despite the slightly higher install cost.