I don't think California can directly mandate use of locomotives meeting a specific emission tier, absent something else going on. Some railroads have agreements (such as the port thing in SoCal) that they will use newer stuff in certain areas. If a railroad uses government money to buy a locomotive (like San Joaquin Valley recently did), they have to be better than basic Tier 4. Then, of course, brand-new locomotives all have to be Tier 4 per EPA (not just California) standards - but there are emission credit programs, based on doing Tier 3 then 4 before it was actually required, that allow continued purchase of or rebuild (of older) to Tier 3 locomotives. So it's kind of a mess, and the only thing for sure is that if somebody buys a new locomotive outside of any emission credit deal it'll be Tier 4.

California has spoken with its (taxpayer) money several times over the years, supporting smaller railroads in getting the best emission performance available, better than standards require if possible. That's sometimes cost-effective from a $/ton of emission reduction perspective. But outright mandate? That was discussed, and was preempted a long time ago by EPA.