According to the U.S. DOE’s Office of Energy Efficiency & Renewable Energy, the U.S. transit bus fleet (as of 2018) is dominated by diesel in various forms - diesel, diesel hybrid or biodiesel. Diesel engines are in around 70% of all transit buses.
More than 20% falls on CNG, LNG and blends. A small slice of over 1% goes to gasoline, while up to around 1% is the "Other" category. There must be some EVs, right?
Hopefully, the progress in bus electrification will gain momentum soon and finally appear on the radar.
"According to the American Public Transit Association (APTA), alternative fuels and advanced hybrid drivetrains powered more than half of all transit buses in 2017 and 2018. In the ten-year period from 2008 to 2018, the share of conventional diesel buses dropped from 70% to 42%. Natural gas and diesel hybrid drivetrains have replaced the greatest share of diesel buses followed by biodiesel and "other" buses, which include hydrogen, electric, and propane."
Source: American Public Transportation Association, 2019 Public Transportation Fact Book, Washington, DC, April 2019, Figure 14.