Arrow in SBD is apparently about to receive its first H2 FLIRT. They started with and currently run a small fleet of Tier-4 DMU versions of the same thing. So you can take a trip to the Inland Empire to see it. Amtrak gets there on the San Joaquin via a bus from Bakersfield to Indio that stops in SBD. Metrolink actually operates the line, and runs peak-time regular commute trains on it, or could (transfer from most commute trains at the downtown SBD station); there allegedly is occasional freight service too chez BNSF.

BOB2 loves the Arrow.

In principle, yes, it's more efficient to run electric trains on wires (minimum loss) and/or batteries (more loss than straight electric, but better than other options). But wires cost a lot to build, and batteries take a long time to recharge even if you have something at each station to boost them a little.

H2/fuel cell trains are attractive because they refuel quickly - as quickly or faster than diesel - and require little if any change to the overall rail infrastructure. It's possible to make "clean" H2 using electricity (presumably sourced from renewables like solar and wind) and water, which SBD has committed to do with their single (so far) train. With fuel cells, the exhaust is ... water. H2 can also be burned in a modified ICE, but in that case does produce a low but measurable level of traditional pollutants chiefly NOx, and it's far less efficient than the fuel cell.