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.