Streetcars are electric, and the majority of buses operate on diesel or gasoline. For cities trying to reduce their carbon footprint it can make an attractive streetcar line. But it is crucial to consider what streetcars are compared to. 
https://www.washingtonpost.com/opinions/riding-light-rail-and-streetcars-into-better-communities/2014/11/14/58fd98b2-6a86-11e4-a31c-77759fc1eacc_story.html