EPA and CARB have to consider the technical feasibility of ways to meet standards they're considering, and have to identify at least one that's feasible both technically and, arguably, cost-effectively. ARB seems to translate that into a mandate for a particular technology more often than not, but they usually back down when alternatives come out of the woodwork.

In this case, I'm sort of amused, because initially the only buses that were considered "zero emission" under the current bus rules were battery and hydrogen (fuel cell) electric. Trolley buses didn't count. Somebody in SF probably leaned on somebody who could tell ARB what to do, so they could get credit for and (maybe someday) expand their trolley bus system. Remember, SF is also unique in the state for its very low-cost power that can only be used for municipal purposes.

When it comes to rail, ARB again ignored (mostly) wired operation, mainly because nobody wanted to do it outside of a few niches for capital and maintenance cost reasons. They didn't ban it; they just assumed that there would be few applications for it, and other ways were needed to power electric transit.

Zero emission if wires are not used devolves to, again, battery and hydrogen fuel-cell. Biofuel doesn't work because while it may be carbon-neutral it still gets burned and emits all that other stuff that has to be controlled. Promoting more wired rail is fine, but the costs don't go away just because it's recognized as ONE OF the ways to meet the rule. The Zero (local) emission mandate can be met in more than one way, and responsible experimentation certainly should be allowed. As with many other things in the world, THERE IS NO SILVER BULLET.