BN Oly, you're right. These are "Federal" sources, covered by Federal Regulations, promulgated by EPA. and CA does not have the authority to ban older locomotives, which can legally operate in CA, as you have observed.

Nitrous oxide emissions were the original problem with locomotives, and when we had no regulations those older, which are kind of nasty, and contribute to Ozone formation. Pre regulation loco's emitted about 10 times as much NOx as today's cleanest standards. More efficient fuel consumption improvements, have reduced some CO2, at the margins, but these Tier's don't do much to reduce CO2 emissions.

As FUD has noted CA has spent a lot over the years to fund research and demonstrations of clean locomotive technology. And, in providing funding to assist in the procurement of new Tier III and Tier IV locomotives.

Since most commuter agencies receive funds to pay for capital equipment with State funding, and are subject to the increasing Federal standards, for new equipment, have also only ordered new locomotives with the level Tier.

The ports are under control of public agencies, and the port RR operators have made various AQ agreements to adopt clean technology, generally as conditions that have been agreed to as part of these broader "clean ports" AQ initiatives to mitigate and reduce aggregate ports related emissions. So yeah, it would be fair to say that where this is the applicable, the RR's are effectively "required" to use only clean locos.

BACT (Best Available Control Technology) for locomotives is ultimately (ZEV), like electrification, which would require a significant investment in electrification infrastructure, and with emerging RR power technology, like that fuel cell powered trolley they want to try out in Santa Cruz. The Germans have been testing fuel cell "DMU's"(of course, in this case, maybe "HMU's" for "hydrogen" would be a more accurate acronym). I believe that Redlands project is looking at this kind of "clean" vehicle. There were some discussions on here a few years back on those German HMU's. None of this technology is a mandated regulatory standard, as of now, that I am aware of.