Although I'm not 100% certain, as I've been out of the game for the last 5 years, and I haven't been able to read the details of this new rulemaking, I think maybe CARB has bitten off a little more than they can chew. While CARB and the USEPA are good buddies and generally work together well, some of us at the staff level would roll our eyes at some of the things they came up with.
One thing that personally affected my work was that they took a voluntary low-rolling-resistance tire program we developed and made it mandatory for everybody (with some exceptions) operating an 18-wheeler truck in California. Essentially, your Federal tax dollars were paying for me to certify tires for the State of California and my agency doesn't even have any legal authority to regulate tires! This program was not designed to meet the normal regulatory requirements, it certainly wasn't a Federal rule; it was devised in cooperation with the tire industry as a marketing tool as part of a larger program to encourage truck fleets and shippers (who we also have no legal authority to regulate) to reduce their emissions. Basically, I spent a lot of my time during the period 2011-2019 processing applications from obscure Chinese tire manufacturers who never would have dreamed of participating in our program if it hadn't been made mandatory by the State of California. And for all that, despite the large amount of testing we did, I couldn't give you a real quantitative estimate of the amount of emissions low rolling resistance tires actually save during actual on-road use. End of my rant. Oh, well, I did get a trip to Beijing out of it, at least.
For both the locomotives and the trucks, this issue seems to be one of having to retire existing equipment before it's ready to be retired and the impact of doing that on the bottom line of the railroads and trucking companies involved. There's also an issue of whether these new "zero-emissions" technologies, by which I guess they mean battery electric trucks and battery electric and fuel cell locomotives, are really as functional and reliable as CARB claims. I suspect that CARB is giving them lots of lead time before all the diesels have to be gone, but that lead time might not be enough. There's also the issue that battery-electric stuff is not "zero emission," as a large portion of electrical power, even in California, still comes from burning coal and natural gas. I hope that CARB has accounted for that in their estimates of emission benefits from their rule.
From my 16 years of experience assessing emissions reduction technologies, I'm not so sure that requiring "zero emissions" is the way to go if the State of California is really interested in reducing total emissions. They might get more benefits from getting a much larger percentage of their population living in walkable communities and seriously decreasing "vehicle miles traveled." They could also change building codes to require more passive heating and cooling of houses and other buildings. And, of course, if "zero emissions" are what you're looking for, maybe it's time to reconsider atomic power, at least for base loads. Also, putting all electric wires underground would go a long way to reduce smoke from wildfires caused by storms knocking down power lines. But do that sort of stuff will require more than a rulemaking by CARB, it will require controversial actions at both the state and local levels from many different agencies of government, who will be fighting heated opposition from businesses and the public, who may say they want to do something about reducing emissions but will balk when they're told what actually needs to be done.