Technicalities: that chart from MDAQMD doesn't make some of the geographic differences clear. In fact, only part of San Bernardino Co (and all of LA Co) is nonattainment for ozone. PM2.5 nonattainment (for federal at least) is limited too. The only federal standard Trona is nonattainment for is the ancient PM10 standard. Since I'm a graphic memory type, I like EPA Region 9's air quality maps at: [www3.epa.gov]

The source of the PM10 (if you want to crawl through the SIP) is primarily the dry Searles Lake bed.

For State standards, Searles is apparently still nonattainment for H2S. There's no federal H2S criteria pollutant standard. That could be from the lake sediments and also from the coal burned at the "trona" processing plant in Trona (as a cogeneration plant, it's the source for essentially all of the power produced in California from coal).

So as Bob says, replacing the Trona Railroad diesels with Tier 3 or 4 would do little or nothing for the emissions causing the area to be listed. The state might have political reasons for doing it, of course.