Here's the amusing part: SF has run zero-emission buses from trolley wires for over 100 years. But, to be a ZEB under the ARB rules, it has to be battery or fuel cell-powered. The regs specifically exempt anything that runs on rails, trolleybuses, and school buses from the requirements, even if operated by a transit agency. So - SF gets no credit for using trolleybuses (they're the only trolley bus operator in CA), even if it expands the system. The only benefit to increasing use of trolley buses would be to reduce diesel bus usage. But the rules don't recognize that; they require replacing diesel with battery or fuel cell buses, not trolley. We can see who backed the regulation, can't we (Proterra, BYD, etc.)? And transit agencies have to work to rule, not to what makes the most sense.

2018 regulations: [ww2.arb.ca.gov]